[time-nuts] Re: Clock specs for audio (was: High precision OCXO supplier for end costomers)

[Magnus Danielson]

Hi,

On 2022-01-10 15:41, Attila Kinali wrote:
> On Mon, 10 Jan 2022 12:35:17 +0100
> Attila Kinali <attila at kinali.ch> wrote:
>
>> That said, are yo sure you need such stringend phase noise requirements?
>> It's audio. Nobody is going to hear whether the noise is -60dBc or -80dBc @ 1Hz,
>> much less -120dBc.
> To give here a bit more background: psychoacoustic masking, which is the relevant
> metric here, mans that we cannot discern sounds that are close to eachother with
> one of them being louder than the other. Depending on who you listen to, it's
> usually a sound masking another sound at a distance of 100Hz up to 20dB to 40dB
> lower. Even if we account for someone with golden ears and use 60dB, that would
> translate to a noise spec of -60dBc @ 100Hz offset. That's a spec that almost all
> XO do fulfill. A good VCXO (40-100MHz) is somewhere around 90-100dBc @ 100Hz.
> Any OCXO will fulfill that spec too, even the tiny DIL-14 ones (most are at
> -110-140dBc @100Hz @10MHz).

A classical paper on jitter for audio is written by late Julian Dunn:

http://www.nanophon.com/audio/jitter92.pdf

He wrote a range of good papers which is worth digging up.

AES-3 (also known as AES/EBU) is the professional version of the digital 
audio interface that in the consumer version is called S/P-DIF. Much of 
the reasoning for AES-3 applies directly onto S/P-DIF.

>
> And this doesn't take into account that we are arguing about audio frequency specs
> at HF frequencies. I.e. if we use the 10MHz clock and use it to derive a sampling
> clock for an ADC to sample a 20kHz signal, the noise performance improves by another
> ~25dB... at least (if the design is done right, it can be up to 50dB)
>
> What is more important than close in noise, though, is broadband noise performance
> and spurs. For someone with good ears, it's not unheard of to be able to discern
> far away noise and spurs down a -100dB-120dB. Especially the spurs can be quite
> hard to control, depending on what clock synthesis system is used.
>
> Another important spec, especially for recording, is accuracy of frequency. An
> offset of just 1ppm becomes 3.6ms if you record for an hour. That's something
> most people can hear already. But whether this actually elevant or not depends
> on how the recording is done. The usual way is to have a central master clock
> that feeds all clocked devices, such that all of them have the same notion of
> time/frequency. In that case, quite high frequency deviations can be tolerated,
> way beyond what a simple XO would deliver.

In professional audio we tolerate +/- 10 ppm for larger productions and 
for all my experience, the exact frequency have not been as much trouble 
as that of different frequencies and the degradation that comes from 
slips, resync or Sample Rate Conversion (SRC). It has been interesting 
to teach the TV and radio techs about basic synchronization and how 
doing it well once save money, time and quality. There is a few war 
stories to be told.

Cheers,
Magnus