Le 11/05/2011 15:10, Invisible a écrit :
> On 11/05/2011 13:21, Le_Forgeron wrote:
>> Le 11/05/2011 10:36, Invisible a écrit :
>>>
>>> I would suggest, however, that 4 bits/sample would be inadequate for
>>> audio applications.
>>
>> For a linear PCM, yes.
> 
> Linear PCM is the standard way to describe digital sound signals.
> 
> I should think that even logarithmic PCM would sound poor at 4 bits/sample.
> 
>> But one of the latest format (SACD) use only 1 bit/sample (at a silly
>> high sampling rate) in a non-PCM storage.
> 
> You mean pulse density modulation.

You can call it like you want. It's a Delta Sigma encoding on 1 bit at
2.8+ MHz: to replay it, you just have to integrate (sigma).

> 
>> (in fact the bit is simply a up/down delta, and it make easy to get ride
>> of the digital to analogue converter in the mass production of player:
>> use the bit to drive digitally a transistor, accumulate output in a
>> capacitor, the connection to the ear-set is done directly on the
>> capacitor (volume is set by resistor on the transistor's upstream))
>> 2 cheap components (or even less if you can fake the capacitor with
>> circuitry) to replace an expensive DAC.
> 
> I'm not sure whether this is true.

It is. At least, it is possible.

> 
> I was also unable to discover what encoding SACD uses for its data. I
> rather doubt that raw PDM written directly to the disc surface, any more
> than a normal CD contains raw PCM.

Raw, no. You need a container for specifying the data rate, the mixing
of the channels. And at lower layer, a bit of self-correcting error
detection encoding (just like the 512/4096 sectors of harddrive are in
fact a bit more bits on the medium, usually with a 8/14 encoding and
some CRC.

A bit like movie containers: the streams must be identified, and
multiplexed. Once separated, each stream can be pushed to its own decoder.

And IIRC, SACD likes to play with DRM (which means a bit of encryption:
the key for the disc is on the disc, but only readable by licensed SACD
device.)

> 
>> Delta Sigma encoding is really painful to convert back accurately in PCM.
> 
> Actually it's trivial. You just need a digital low-pass filter.

The issue is computing the right parameters of the filter and
*accurately*. (If many PCM to/from SACD encoding+decoding was performed
in sequences, some difference might appears between the various PCM
stages... ). Fine tuning that part was really out of my capability.

-- 
Software is like dirt - it costs time and money to change it and move it
around.<br/><br/>


Just because you can't see it, it doesn't weigh anything,
and you can't drill a hole in it and stick a rivet into it doesn't mean
it's free.

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