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    <B>Peter Mapp</B> 

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    <I>Fri Jan 13 13:01:26 EST 2017</I>

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<PRE>Thanks Charles. You have answered the point I was trying to get at

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Robinson, Charles Q via SC-04-08

Sent: 13 January 2017 17:49

To: Working group on Measurement and equalization of sound systems in rooms

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

><i> Are the errors the same for a DFT or just applicable to FFT ?

</I>

The term "FFT" is often used synonymously with DFT.  Other times it is thought that an FFT is a kind of DFT approximation.  In fact FFT means "an efficient implementation of a DFT."  Mathematically, FFTs are an exact factorization of the DFT.  (This is analogous to splitting a high-order filter into a cascade of second order filters.) The specific formulation of an FFT can introduce constraints (e.g. power of 2 block length), but an FFT does not introduce errors relative to a "direct," brute-force DFT computation.  (Both an FFT and a brute-force DFT computed numerically will introduce quantization errors).

​

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of John Woodgate via SC-04-08

Sent: Friday, January 13, 2017 7:42 AM

To: 'Peter Mapp'; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

I don't know whether you can get an accurate  FFT or DFT for an MLS, but I expect Thomas Lagõ does.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 3:35 PM

To: John Woodgate <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>>; 'Working group on Measurement and equalization of sound systems in rooms' <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

This suggests then that you could use a sine sweep / esweep but not pink noise. Maximal length sequence ?

​

pm

​

-----Original Message-----

From: John Woodgate [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>]

Sent: 13 January 2017 15:17

To: Peter Mapp; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: RE: [SC-04-08] Comments uploaded by David Murphy

​

They are inherent in the math of Fourier analysis, so I suppose they affect any implementation. But there are some clever modifications of the algorithms that claim to reduce some types of error, such as those due to not having an even number of samples in the measurement interval. I don't think they can be eliminated.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:50 PM

To: John Woodgate <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>>; 'Working group on Measurement and equalization of sound systems in rooms' <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Thanks John

​

Are the errors the same for a DFT or just applicable to FFT ?

​

pm

​

-----Original Message-----

From: John Woodgate [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>]

Sent: 13 January 2017 14:46

To: Peter Mapp; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: RE: [SC-04-08] Comments uploaded by David Murphy

​

The errors could be very large if FFT is incorrectly applied, as Thomas at least hints at in 'stationary and discrete'. Of course, you know this, but many do not.

​

The most obvious source of error is using a measurement interval that does not embrace an exact number of cycles of every frequency present in the signal. This is what can be 'reduced' by windowing, but that actually disguises the error rather than correcting it. One such effect is the appearance in the FFT of a signal at a low frequency, which may be of quite significant amplitude, and a long succession of its harmonics that are not present at all in the real signal. It's possible to get this effect even when measuring an amplifier, where there is no possible doubt that the spurious signal is not present in reality.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:28 PM

To: Thomas Lagö <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">thomaslagoqirra at gmail.com</A>>; Working group on Measurement and equalization of sound systems in rooms <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>; David Murphy <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com.au</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Whilst what you say is of course true Thomas, the question is what in practice, when measuring a typical cinema system, would the errors be and are they actually significant within this context  ?

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Thomas Lagö via SC-04-08

Sent: 12 January 2017 12:39

To: David Murphy; Charlie Hughes via SC-04-08

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for 

</I>><i> SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>&g...

​

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>Thomas Lagö</B> 

    <A HREF="mailto:sc-04-08%40standards.aes.org?Subject=Re%3A%20%5BSC-04-08%5D%20Comments%20uploaded%20by%20David%20Murphy&In-Reply-To=%3C4B8B84A4-9ECD-45BF-A864-F94F3B53C24C%40msn.com%3E" TITLE="[SC-04-08] Comments uploaded by David Murphy">thomaslagoqirra at gmail.com

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<PRE>Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>><i> Concern about hearing protection if operator is in theatre – to chase down rattles and buzzes using frequency sweeps at -3dBFS (or -10dBFS) will be very loud, exceeds operator’s noise dose? 

</I>><i> Concern about loose terminology for measuring variations of SPL in seating area – needs more specificity. Loose terminology also re listening test evaluation

</I>><i> Fraught suggestions for verifying crossover integrity of screen loudspeakers. refer uploaded doc for my suggestions!

</I>><i> In general the document is a bit Panglossian, assuming that everything is best practice, such as infinite baffle mounting of screen loudspeaker systems, well behaved loudspeaker systems, lots of absorption everywhere, well behaved reflections and RT60, etc. 

</I>><i> I think it would be good to give some hints or guidance on what to do if any or all of these conditions are not present. For example, how to select appropriate window length and shape for the measurement.

</I>><i> 

</I>><i> best regards

</I>><i> David

</I>><i> 

</I>><i> David Murphy 

</I>><i> Loudspeaker Design Engineer

</I>><i> Commercial Cinema

</I>><i> 

</I>><i> Krix 

</I>><i> 14 Chapman Road | Hackham SA 5163 | Australia 

</I>><i> T +618 83843433 | F +618 83843419

</I>><i> Email: <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com</A> | Web: www.krix.com

</I>><i> 

</I>><i> 

</I>><i> 

</I>><i> DISCLAIMER:

</I>><i> This email and any files transmitted with it are confidential to the intended recipient and may be privileged. If you are not the intended recipient, please notify Krix immediately. Whilst we have used software to alert us to the presence of computer viruses, we cannot guarantee that this email and any files transmitted with it are free from them.

</I>><i> _______________________________________________

</I>><i> SC-04-08 mailing list

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</I>

</PRE>

​

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>Peter Mapp</B> 

    <A HREF="mailto:sc-04-08%40standards.aes.org?Subject=Re%3A%20%5BSC-04-08%5D%20Comments%20uploaded%20by%20David%20Murphy&In-Reply-To=%3Cabc296d866ba4b6eb8923aa1fee72906%40CP1-L1-MB-2003.CloudPlatform1.com%3E" TITLE="[SC-04-08] Comments uploaded by David Murphy">peter at petermapp.com

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<PRE>Whilst what you say is of course true Thomas, the question is what in practice, when measuring a typical cinema system, would the errors be and are they actually significant within this context  ?

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Thomas Lagö via SC-04-08

Sent: 12 January 2017 12:39

To: David Murphy; Charlie Hughes via SC-04-08

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for 

</I>><i> SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>><i> Concern about hearing protection if operator is in theatre – to chase down rattles and buzzes using frequency sweeps at -3dBFS (or -10dBFS) will be very loud, exceeds operator’s noise dose? 

</I>><i> Concern about loose terminology for measuring variations of SPL in 

</I>><i> seating area – needs more specificity. Loose terminology also re listening test evaluation Fraught suggestions for verifying crossover integrity of screen loudspeakers. refer uploaded doc for my suggestions!

</I>><i> In general the document is a bit Panglossian, assuming that everything is best practice, such as infinite baffle mounting of screen loudspeaker systems, well behaved loudspeaker systems, lots of absorption everywhere, well behaved reflections and RT60, etc. 

</I>><i> I think it would be good to give some hints or guidance on what to do if any or all of these conditions are not present. For example, how to select appropriate window length and shape for the measurement.

</I>><i> 

</I>><i> best regards

</I>><i> David

</I>><i> 

</I>><i> David Murphy

</I>><i> Loudspeaker Design Engineer

</I>><i> Commercial Cinema

</I>><i> 

</I>><i> Krix

</I>><i> 14 Chapman Road | Hackham SA 5163 | Australia T +618 83843433 | F +618 

</I>><i> 83843419

</I>><i> Email: <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com</A> | Web: www.krix.com

</I>><i> 

</I>><i> 

</I>><i> 

</I>><i> DISCLAIMER:

</I>><i> This email and any files transmitted with it are confidential to the intended recipient and may be privileged. If you are not the intended recipient, please notify Krix immediately. Whilst we have used software to alert us to the presence of computer viruses, we cannot guarantee that this email and any files transmitted with it are free from them.

</I>><i> _______________________________________________

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_______________________________________________

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>John Woodgate</B> 

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<PRE>The errors could be very large if FFT is incorrectly applied, as Thomas at least hints at in 'stationary and discrete'. Of course, you know this, but many do not.

​

The most obvious source of error is using a measurement interval that does not embrace an exact number of cycles of every frequency present in the signal. This is what can be 'reduced' by windowing, but that actually disguises the error rather than correcting it. One such effect is the appearance in the FFT of a signal at a low frequency, which may be of quite significant amplitude, and a long succession of its harmonics that are not present at all in the real signal. It's possible to get this effect even when measuring an amplifier, where there is no possible doubt that the spurious signal is not present in reality.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only

www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:28 PM

To: Thomas Lagö <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">thomaslagoqirra at gmail.com</A>>; Working group on Measurement and equalization of sound systems in rooms <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>; David Murphy <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com.au</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Whilst what you say is of course true Thomas, the question is what in practice, when measuring a typical cinema system, would the errors be and are they actually significant within this context  ?

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Thomas Lagö via SC-04-08

Sent: 12 January 2017 12:39

To: David Murphy; Charlie Hughes via SC-04-08

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for 

</I>><i> SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>><i> Concern about hearing protection if operator is in theatre – to chase down rattles and buzzes using frequency sweeps at -3dBFS (or -10dBFS) will be very loud, exceeds operator’s noise dose? 

</I>><i> Concern about loose terminology for measuring variations of SPL in 

</I>><i> seating area – needs more specificity. Loose terminology also re listening test evaluation Fraught suggestions for verifying crossover integrity of screen loudspeakers. refer uploaded doc for my suggestions!

</I>><i> In general the document is a bit Panglossian, assuming that everything is best practice, such as infinite baffle mounting of screen loudspeaker systems, well behaved loudspeaker systems, lots of absorption everywhere, well behaved reflections and RT60, etc. 

</I>><i> I think it would be good to give some hints or guidance on what to do if any or all of these conditions are not present. For example, how to select appropriate window length and shape for the measurement.

</I>><i> 

</I>><i> best regards

</I>><i> David

</I>><i> 

</I>><i> David Murphy

</I>><i> Loudspeaker Design Engineer

</I>><i> Commercial Cinema

</I>><i> 

</I>><i> Krix

</I>><i> 14 Chapman Road | Hackham SA 5163 | Australia T +618 83843433 | F +618 

</I>><i> 83843419

</I>><i> Email: <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com</A> | Web: www.krix.com

</I>><i> 

</I>><i> 

</I>><i> 

</I>><i> DISCLAIMER:

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>John Woodgate</B> 

    <A HREF="mailto:sc-04-08%40standards.aes.org?Subject=Re%3A%20%5BSC-04-08%5D%20Comments%20uploaded%20by%20David%20Murphy&In-Reply-To=%3C005101d26db0%241d2c2630%2457847290%24%40btinternet.com%3E" TITLE="[SC-04-08] Comments uploaded by David Murphy">jmw1937 at btinternet.com

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<PRE>They are inherent in the math of Fourier analysis, so I suppose they affect any implementation. But there are some clever modifications of the algorithms that claim to reduce some types of error, such as those due to not having an even number of samples in the measurement interval. I don't think they can be eliminated.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only

www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:50 PM

To: John Woodgate <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>>; 'Working group on Measurement and equalization of sound systems in rooms' <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Thanks John

​

Are the errors the same for a DFT or just applicable to FFT ?

​

pm

​

-----Original Message-----

From: John Woodgate [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>] 

Sent: 13 January 2017 14:46

To: Peter Mapp; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: RE: [SC-04-08] Comments uploaded by David Murphy

​

The errors could be very large if FFT is incorrectly applied, as Thomas at least hints at in 'stationary and discrete'. Of course, you know this, but many do not.

​

The most obvious source of error is using a measurement interval that does not embrace an exact number of cycles of every frequency present in the signal. This is what can be 'reduced' by windowing, but that actually disguises the error rather than correcting it. One such effect is the appearance in the FFT of a signal at a low frequency, which may be of quite significant amplitude, and a long succession of its harmonics that are not present at all in the real signal. It's possible to get this effect even when measuring an amplifier, where there is no possible doubt that the spurious signal is not present in reality.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:28 PM

To: Thomas Lagö <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">thomaslagoqirra at gmail.com</A>>; Working group on Measurement and equalization of sound systems in rooms <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>; David Murphy <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com.au</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Whilst what you say is of course true Thomas, the question is what in practice, when measuring a typical cinema system, would the errors be and are they actually significant within this context  ?

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Thomas Lagö via SC-04-08

Sent: 12 January 2017 12:39

To: David Murphy; Charlie Hughes via SC-04-08

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for 

</I>><i> SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>><i> Concern about hearing protection if operator is in theatre – to chase down rattles and buzzes using frequency sweeps at -3dBFS (or -10dBFS) will be very loud, exceeds operator’s noise dose? 

</I>><i> Concern about loose terminology for measuring variations of SPL in 

</I>><i> seating area – needs more specificity. Loose terminology also re listening test evaluation Fraught suggestions for verifying crossover integrity of screen loudspeakers. refer uploaded doc for my suggestions!

</I>><i> In general the document is a bit Panglossian, assuming that everything is best practice, such as infinite baffle mounting of screen loudspeaker systems, well behaved loudspeaker systems, lots of absorption everywhere, well behaved reflections and RT60, etc. 

</I>><i> I think it would be good to give some hints or guidance on what to do if any or all of these conditions are not present. For example, how to select appropriate window length and shape for the measurement.

</I>><i> 

</I>><i> best regards

</I>><i> David

</I>><i> 

</I>><i> David Murphy

</I>><i> Loudspeaker Design Engineer

</I>><i> Commercial Cinema

</I>><i> 

</I>><i> Krix

</I>><i> 14 Chapman Road | Hackham SA 5163 | Australia T +618 83843433 | F +618

</I>><i> 83843419

</I>><i> Email: <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com</A> | Web: www.krix.com

</I>><i> 

</I>><i> 

</I>><i> 

</I>><i> DISCLAIMER:

</I>><i> This email and any files transmitted with it are confidential to the intended recipient and may be privileged. If you are not the intended recipient, please notify Krix immediately. Whilst we have used software to alert us to the presence of computer viruses, we cannot guarantee that this email and any files transmitted with it are free from them.

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</I>><i> SC-04-08 mailing list

</I>><i> <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">SC-04-08 at standards.aes.org</A>

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>Peter Mapp</B> 

    <A HREF="mailto:sc-04-08%40standards.aes.org?Subject=Re%3A%20%5BSC-04-08%5D%20Comments%20uploaded%20by%20David%20Murphy&In-Reply-To=%3Ce9f19f68fa784d3ba0b6846f4b9b6014%40CP1-L1-MB-2003.CloudPlatform1.com%3E" TITLE="[SC-04-08] Comments uploaded by David Murphy">peter at petermapp.com

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<PRE>Thanks John

​

Are the errors the same for a DFT or just applicable to FFT ?

​

pm

​

-----Original Message-----

From: John Woodgate [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>] 

Sent: 13 January 2017 14:46

To: Peter Mapp; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: RE: [SC-04-08] Comments uploaded by David Murphy

​

The errors could be very large if FFT is incorrectly applied, as Thomas at least hints at in 'stationary and discrete'. Of course, you know this, but many do not.

​

The most obvious source of error is using a measurement interval that does not embrace an exact number of cycles of every frequency present in the signal. This is what can be 'reduced' by windowing, but that actually disguises the error rather than correcting it. One such effect is the appearance in the FFT of a signal at a low frequency, which may be of quite significant amplitude, and a long succession of its harmonics that are not present at all in the real signal. It's possible to get this effect even when measuring an amplifier, where there is no possible doubt that the spurious signal is not present in reality.

​

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:28 PM

To: Thomas Lagö <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">thomaslagoqirra at gmail.com</A>>; Working group on Measurement and equalization of sound systems in rooms <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>; David Murphy <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com.au</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Whilst what you say is of course true Thomas, the question is what in practice, when measuring a typical cinema system, would the errors be and are they actually significant within this context  ?

​

Peter

​

-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Thomas Lagö via SC-04-08

Sent: 12 January 2017 12:39

To: David Murphy; Charlie Hughes via SC-04-08

Subject: Re: [SC-04-08] Comments uploaded by David Murphy

​

Dear All,

​

The FFT is not a good tool when intersted in amplitude accuracy unless the signal is known and proper scaling and windowing has been applied. Also, "allowing windows" is not a technology advance. It is a necessary tool to handle leakage! For most cases, the Hanning window will be the best one but Flattop can be an alternative given that we know more about the signal. Please note, that FFT requires stationary and discrete signals (that can be described by a Fourier series, that is).

​

I have published many papers on this topic and I often see the FFT being misused (and abused) in audio engineering, sorry!

​

Best regards,

Thomas Lagö

​

><i> On 12 Jan 2017, at 12:48, david murphy via SC-04-08 <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>> wrote:

</I>><i> 

</I>><i> Hi all,

</I>><i> I have just uploaded my comments to the upload/download website for 

</I>><i> SC-04-08, named SMPTE_2096-1_Comment_David_Murphy_Jan_2017

</I>><i> 

</I>><i> A very short summary follows

</I>><i> I note that that in 2. Introduction: end of para 4 “….The suite does not define new standards or supercede current ones, but offers current approaches and methodology using modern equipment to more accurately calibrate within the tolerances of the existing standards.”

</I>><i> Accordingly I will try to comment only on the methodology, and hope a better frequency response standard than the X Curve Standard can be developed.

</I>><i> 

</I>><i> Worthwhile improvement, more use cases for microphone placement, specifying target area for frequency response and including LFE as well.

</I>><i> Big advance is using FFT measuring system allowing windowing etc.

</I>><i> Concern about hearing protection if operator is in theatre – to chase down rattles and buzzes using frequency sweeps at -3dBFS (or -10dBFS) will be very loud, exceeds operator’s noise dose? 

</I>><i> Concern about loose terminology for measuring variations of SPL in 

</I>><i> seating area – needs more specificity. Loose terminology also re listening test evaluation Fraught suggestions for verifying crossover integrity of screen loudspeakers. refer uploaded doc for my suggestions!

</I>><i> In general the document is a bit Panglossian, assuming that everything is best practice, such as infinite baffle mounting of screen loudspeaker systems, well behaved loudspeaker systems, lots of absorption everywhere, well behaved reflections and RT60, etc. 

</I>><i> I think it would be good to give some hints or guidance on what to do if any or all of these conditions are not present. For example, how to select appropriate window length and shape for the measurement.

</I>><i> 

</I>><i> best regards

</I>><i> David

</I>><i> 

</I>><i> David Murphy

</I>><i> Loudspeaker Design Engineer

</I>><i> Commercial Cinema

</I>><i> 

</I>><i> Krix

</I>><i> 14 Chapman Road | Hackham SA 5163 | Australia T +618 83843433 | F +618

</I>><i> 83843419

</I>><i> Email: <A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">dmurphy at krix.com</A> | Web: www.krix.com

</I>><i> 

</I>><i> 

</I>><i> 

</I>><i> DISCLAIMER:

</I>><i> This email and any files transmitted with it are confidential to the intended recipient and may be privileged. If you are not the intended recipient, please notify Krix immediately. Whilst we have used software to alert us to the presence of computer viruses, we cannot guarantee that this email and any files transmitted with it are free from them.

</I>><i> _______________________________________________

</I>><i> SC-04-08 mailing list

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   <H1>[SC-04-08] Comments uploaded by David Murphy</H1>

    <B>Peter Mapp</B> 

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    <I>Fri Jan 13 10:34:43 EST 2017</I>

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<PRE>This suggests then that you could use a sine sweep / esweep but not pink noise. Maximal length sequence ?

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pm

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-----Original Message-----

From: John Woodgate [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>] 

Sent: 13 January 2017 15:17

To: Peter Mapp; 'Working group on Measurement and equalization of sound systems in rooms'

Subject: RE: [SC-04-08] Comments uploaded by David Murphy

​

They are inherent in the math of Fourier analysis, so I suppose they affect any implementation. But there are some clever modifications of the algorithms that claim to reduce some types of error, such as those due to not having an even number of samples in the measurement interval. I don't think they can be eliminated.

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With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

​

Sylvae in aeternum manent.

​

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-----Original Message-----

From: SC-04-08 [mailto:<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08-bounces at standards.aes.org</A>] On Behalf Of Peter Mapp via SC-04-08

Sent: Friday, January 13, 2017 2:50 PM

To: John Woodgate <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">jmw1937 at btinternet.com</A>>; 'Working group on Measurement and equalization of sound systems in rooms' <<A HREF="https://secure.aes.org/mailman/listinfo/sc-04-08">sc-04-08 at standards.aes.org</A>>

Subject: Re: [SC-04-08] Comments uploaded by David Murphy