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Slew rate limit for SACD
Slew rate limit for SACD
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Old 23rd October 2006, 06:29 PM   #11
peufeu is offline peufeu  France
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Location: Lyon, France
Whan I would really like to test would be this :

Imagine a test setup with a multibit DAC like TDA1543 feeding a sigma-delta ADC which outputs a DSD stream.

We can then feed test signals and record the DSD stream.

This is just like recording a musical performance to DSD, except said performance will be test signals produced by the DAC.

Now, have the DAC output a test signal which would be a single sample pulse of known amplitude.

This is the absolute opposite of steady-state sinewave testing.

Then, look at the DSD output in a time window surrounding the test pulse. Suppose you look from the start of the pulse and during 2 PCM samples, this means 45 us, or 128 DSD pulses.

Now do a bit of information theory. Is DSD capable to losslessly encode PCM ? This can be easily tested. Just send pulses of all the possible amplitudes (65536 for 16 bits). Maybe send each pulse 64 times or something. Each time, record the DSD stream.

Now, if DSD can record PCM losslessly, all PCM pulses should correspond to a different DSD encoding. In this case DSD would win.

However, if several PCM pulses end up with the same DSD encoding, when replaying the DSD stream it is impossible to reconstruct the original PCM values. Thus, DSD would lose.

Personnaly I think DSD would lose big time. If it didn't, then why have all manufacturers droppped 1-bit DACs long ago and generally use multibit (generally 6 bits) sigma delta running at 6 or more MHz ?

This test can be performed in a perfectly exact way in simulation if someone can get me the equations for a DSD encoder.

Anyone ?
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Old 23rd October 2006, 06:41 PM   #12
BlackCatSound is offline BlackCatSound  United Kingdom
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DSD can slew from one end of its scale to the other in 1 sample, as can all 1 bit systems.

The stream simple goes 000000001111111111 (or the other way) and you get a step change.

The limiting factor is the analogue filtering that takes out all the HF from this signal and just passes the audio freq component.
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