[time-nuts] Oh dear

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Mon May 7 16:31:10 UTC 2012


If you take into consideration that the best currently available DACs, 
also true for analog circuits, have a dynamic range about 120-126dB, the 
last 3-4 bits are quite irrelevant (random noise mostly)... a good 20bit 
DAC already pushes the limits.
The marketingdroids swarming for the newest "32" bitters is even more 
ludicrous.
On the other side, the dynamic range of the ear (if you care the least 
for the future of your hearing), and of the quietest available listening 
spaces, hardly gets to 100dB...

Of course, for the DSPs involved in the signal chain, 32bits integer 
math might not be enough, due to rounding errors.

On 5/7/2012 7:02 PM, Tom Knox wrote:
>
> Actually the numbers are quite real, play with the math, a small amount of jitter in a DAC (X) can
> have a large difference (Y) when sampling a complex wave form especially
>   in the audiophile world where the sound of 24bit dac 16,777,216 discrete levels is clearly superior to older 16 bit dac 65,536 possible levels in 44.1 KHz to 192 KHz formats.
>
> Thomas Knox
>
>
>
>> Date: Mon, 7 May 2012 17:59:04 +0200
>> From: attila at kinali.ch
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] Oh dear
>>
>> On Mon, 07 May 2012 08:20:55 -0700
>> Dan Rae<danrae at verizon.net>  wrote:
>>
>>> I see nothing odd about wanting to get the best possible source for the
>>> Master Clock for your master recordings.
>>>
>>> My son does run a small studio and for him I was able to make a version
>>> of that unit, for a lot less money of course.  If he says it improves
>>> the sound of the recordings, and his customers agree, I am inclined to
>>> believe him.
>>
>> The thing is, that an Rb is good for one thing: Have a long term
>> stable and accurate frequency source that is better than 1 to some
>> billions for measurement or other stuff that take more than a few
>> hours or have to be repeated exactly in a couple of weeks.
>>
>> For audio, you need a frequency source that is stable over a couple
>> of hours (probably a working day) and shows "low" jitter. Where as low
>> jitter is quite high in time-nuts terms and stable not stable at all.
>> A cycle-to-cylcle jitter of a couple of ns is not audioable at all,
>> but any Rb will have a much lower jitter. Or to have a different look at it,
>> you want to have very low phase noise, as this phase noise is mixed in
>> over the ADCs into your signal. But as we know, the phase noise of
>> an Rb is not defined by the Rb physics package, but by the OCXO they use.
>> (yes i know that the close in phase noise is defined by the reference
>> and not by the OCXO, but the "base level" is the OCXO, not the reference)
>>
>> As for stability. You want the instruments to sound the same over an
>> recording. Ie the human ear has to preceive the recorded sound as the
>> same. The frequency resolution of the human ear is somewhere around 3Hz.
>> This makes for 150ppm (at 20kHz). Even a 32kHz tuning fork crystal
>> achieves an absolute accuracy that is better than this. Its stability is much
>> better than this....
>> Of course, you want to have enought headroom for other non ideal components.
>> So, lets say, go for a factor of 10, then we are at 15ppm. For absolute
>> accuracy, that's already a good XO. For stability, still most XO should
>> do that.
>>
>> Or to say it differently: Using some good OCXO with low or very low
>> phase noise would be more than enough for even the most high end
>> audio equipment. You don't even have to discipline it, as a even
>> quite bad OCXO has variations much lower than 1ppm, which is definitly
>> not something anyone can hear.
>>
>> IMHO getting a 20-50USD OCXO from ebay, some good, low noise power supply
>> (audio power supplies with low noise in the<40kHz region), some distribution
>> amplifier with low noise figure and you are set. All in all probably at
>> a cost of 200-300USD including rack mount. If you want to have "high fidelity"
>> you can use an GPSDO to get your OCXO within a couple mHz.
>>
>> To summarize: Nobody here does want to insult anyone who does professional
>> audio recordings. But having the knowledge of what the stability and
>> accuracy numbers for an ordinary Rb mean, and being able to put that into
>> perspective with the not so good capabilties of the human sensory systems,
>> one wonders why people spend an awfull lot of money for something that has
>> no audiable effect over something a lot cheaper. Not to mention that other
>> things have a much higher impact on audio quality than the reference
>> oscillator: Like temperature and humidity during recording (do you control
>> them as well to the ppm level?), or the tuning of the instruments which
>> wanders quite a bit during use.
>>
>> 				Attila Kinali
>>
>> --
>> Why does it take years to find the answers to
>> the questions one should have asked long ago?
>>
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