Prev: Changing Potentiometer Value
Next: Tiller's "Intention Imprinting" Device?
From: John Larkin on 29 May 2010 01:33
On Fri, 28 May 2010 21:35:38 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Fri, 28 May 2010 21:10:32 -0700 (PDT), Glenn Gundlach
><stratus46(a)yahoo.com> wrote:
>
>>There is a guy on DIY audio stating that jitter in SPDIF digital audio
>>causes distortions. OK. He also claims to have jitter in the 100 fs
>>area. Is he blowing smoke about the jitter or can he really achieve
>>sub pico second jitter? It seems to me that jitter in that system is
>>similar to Time Base errors in video systems. In analog video 1-2 nS
>>was achieved in both analog and digital Time Base Correction (TBC)
>>systems but I'm having problems getting my head around 100 fS and - is
>>it REALLY audible?
>>
>>Thanks to all for any insight.
>>
>>G�
>
>100 fs RMS cycle-cycle jitter is possible from a very good, pretty
>expensive crystal oscillator. Of course it's not audible.
>
>Any signal processing, including quantization, causes distortion. So
>do microphones, speakers, and ears. People who worry about
>femtoseconds are lunatics.
>
>Sound doesn't travel very far in 100 fs. The performer's guitar, the
>recording microphone, the speaker, the walls of your listening room,
>your eardrums... all are moving around constantly much, much more than
>the distance sound travels in 100 fs. One *microinch* of vibration of
>any of them corresponds to 30 ns of jitter.

No, cancel that. A microinch is about 77 psec.

John

From: Vladimir Vassilevsky on 29 May 2010 01:37


Glenn Gundlach wrote:

> There is a guy on DIY audio stating that jitter in SPDIF digital audio
> causes distortions. OK. He also claims to have jitter in the 100 fs
> area. Is he blowing smoke about the jitter or can he really achieve
> sub pico second jitter?

Sub-picosecond jitter is non-trivial.

Jitter = Noise/Slew_rate

So, for the reasonable slew rates ~1e9 V/s the noise should be ~1uV.
This requires narrow band high frequency design.

> It seems to me that jitter in that system is
> similar to Time Base errors in video systems. In analog video 1-2 nS
> was achieved in both analog and digital Time Base Correction (TBC)
> systems but I'm having problems getting my head around 100 fS and - is
> it REALLY audible?


S/(THD + N) impairment = Jitter x Frequency x 2Pi/sqrt(3)

For 20kHz and 120dB, jitter should be ~ 10 pS. There is absolutely no
point in pushing into femtosecond area.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: MooseFET on 29 May 2010 11:39
On May 28, 9:10 pm, Glenn Gundlach <stratu...(a)yahoo.com> wrote:
> There is a guy on DIY audio stating that jitter in SPDIF digital audio
> causes distortions. OK. He also claims to have jitter in the 100 fs
> area. Is he blowing smoke about the jitter or can he really achieve
> sub pico second jitter? It seems to me that jitter in that system is
> similar to Time Base errors in video systems. In analog video 1-2 nS
> was achieved in both analog and digital Time Base Correction (TBC)
> systems but I'm having problems getting my head around 100 fS and - is
> it REALLY audible?
>
> Thanks to all for any insight.

The jitter in the clock does not really add distortion. It adds a
signal
dependent noise. The noise in the data depends on the clock jitter
and
the slew rate of the input.

The jitter than really matters is the portion that makes things that
can be heard in the output. This biases it towards the near carrier
noise where the noise is usually the worst. I know from experience
that
in the near carrier case, an OCXO can be over 180dB down.
From: Stephan Goldstein on 29 May 2010 20:18
On Sat, 29 May 2010 00:37:13 -0500, Vladimir Vassilevsky
<nospam(a)nowhere.com> wrote:

>
>
>Glenn Gundlach wrote:
>
>> There is a guy on DIY audio stating that jitter in SPDIF digital audio
>> causes distortions. OK. He also claims to have jitter in the 100 fs
>> area. Is he blowing smoke about the jitter or can he really achieve
>> sub pico second jitter?
>
>Sub-picosecond jitter is non-trivial.
>
>Jitter = Noise/Slew_rate
>
>So, for the reasonable slew rates ~1e9 V/s the noise should be ~1uV.
>This requires narrow band high frequency design.
>
>> It seems to me that jitter in that system is
>> similar to Time Base errors in video systems. In analog video 1-2 nS
>> was achieved in both analog and digital Time Base Correction (TBC)
>> systems but I'm having problems getting my head around 100 fS and - is
>> it REALLY audible?
>
>
>S/(THD + N) impairment = Jitter x Frequency x 2Pi/sqrt(3)
>
>For 20kHz and 120dB, jitter should be ~ 10 pS. There is absolutely no
>point in pushing into femtosecond area.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com

A coworker has been concerned about jitter-induced degradation in
a data acquisition system and has been trying to find a simple
equation like this one. Can you give me a reference I can pass
along to him? He's an analog-circuits guy like myself, and not
terribly familiar with the digital literature.

Thanks.
From: Paul Keinanen on 30 May 2010 03:46
On Fri, 28 May 2010 21:10:32 -0700 (PDT), Glenn Gundlach
<stratus46(a)yahoo.com> wrote:

>There is a guy on DIY audio stating that jitter in SPDIF digital audio
>causes distortions. OK.


This seemed to be a popular subject for more than a decade ago,
someone are still trying to make money on this :-).

Apparently some early external DACs took the self clocking Manchester
code SPDIF signal, passed it through a comparator and use the
transitions to kick a multivibrator, which was divided down to
directly drive the DAC sample clock.

Of course, the previous data contents in not so linear phase transfer
path or some added hum in coaxial versions shifted the threshold level
of the digital data, causing some timing jitter. Even after dividing
down this to the audio sampling rate, the sample pulse timing contains
some jitter.

The sampling pulse jitter will cause some amplitude distortion in the
presence of high amplitude high voltage signals (high slave rate). For
audio DACs the largest slave rate is during the zero crossing of a
full amplitude 20 kHz sine wave.

From the slave rate it can be calculated, how much the sample point
must be off to produce say 1 LSB error. This still might be audible,
since low frequency (< 1kHz) tones can be detected down to -110m ..
-120 dB levels in fade to noise tests due to dithering, even if the 16
bit CD system can go only down to -96 dB for any frequency.

Thus -120 dB or 1/16 LSB (of original 16 bit signal) should be quite
safe, especially when 20 kHz 0 dB signals are _quite_ rare in any
actual recordings :-), reducing the maximum realistic slew rate with
an order of magnitude.

In any modern SPDIF receivers, some kind of PLL is used and the loop
filter removes any SPDIF receiver comparator threshold related jitter,
so the external DAC jitter should not have been an issue for a very
long time.

First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4
Prev: Changing Potentiometer Value
Next: Tiller's "Intention Imprinting" Device?