Re: pазpядность ЦАП

From

Anton Osennikov (2:5054/1.128)

To

George Podukov (2:5054/37.63)

Date

2005-04-27T11:48:52Z

Area

PERM.HARDW

From: Anton Osennikov <ant@perm.ru>

George Podukov wrote:

>  AO> Это другие биты. ЦАП любой разрядности (нынче обычно 16 или 24) можно
>  AO> построить  однобитным или мультибитным.
> Где про это почитать?

Дык смотря на каком уровне (и языке ;)

Под рукой сейчас FAQ:

What are the differences between multibit and Bitstream/MASH Analogue to 
Digital converters (16-bit vs 1-bit CD players)?
Audio data is stored on CD as 16-bit words. It is the job of the digital to 
analogue converter (DAC) to convert these numbers to a varying voltage. Many 
DAC chips do this by storing electric charge in capacitors (like water in 
buckets) and selectively emptying these buckets to the analogue ouput, thereby 
adding their contents. Others sum the outputs of current or voltage sources, 
but the operating principles are otherwise similar.

A multi-bit converter has sixteen buckets corresponding to the sixteen bits of 
the input word, and sized 1, 2, 4, 8 ... 32768 charge units. Each word (ie 
sample) decoded from the disc is passed directly to the DAC, and those buckets 
corresponding to 1's in the input word are emptied to the output.

To perform well the bucket sizes have to be accurate to within +/- half a 
charge unit; for the larger buckets this represents a tolerance tighter than 
0.01%, which is difficult. Furthermore the image spectrum from 24kHz to 64kHz 
must be filtered out, requiring a complicated, expensive filter.

Alternatively, by using some digital signal processing, the stream of 16-bit 
words at 44.1kHz can be transformed to a stream of shorter words at a higher 
rate. The two data streams represent the same signal in the audio band, but the 
new data stream has a lot of extra noise in it resulting from the word length 
reduction. This extra noise is made to appear mostly above 20kHz through the 
use of noise-shaping, and the oversampling ensures that the first image 
spectrum occurs at a much higher frequency than in the multi-bit case.

This new data stream is now converted to an analogue voltage by a DAC of short 
word length; subsequently, most of the noise above 20kHz can be filtered out by 
a simple analogue filter without affecting the audio signal.

Typical configurations use 1-bit words at 11.3MHz (256 times over-sampled), and 
4-bit words at 2.8MHz (64 times oversampled). The former requires one bucket of 
arbitrary size (very simple); it is the basis of the Philips Bitstream range of 
converters. The latter requires four buckets of sizes 1, 2, 4 and 8 charge 
units, but the tolerance on these is relaxed to about 5%.

MASH and other PWM systems are similar to Bitstream, but they vary the pulse 
width at the ouput of the digital signal processor. This can be likened to 
using a single bucket but with the provision to part fill it. For example, MASH 
allows the bucket to be filled to eleven different depths (this is where they 
get 3.5 bits from, as 2^(3.5) is approximately eleven).

Lastly it is important to note that these are all simply different ways of 
performing the same function. It is easy to make a lousy CD player based around 
any of these technologies; it is rather more difficult to make an excellent 
one, regardless of the DAC technology employed. Each of the conversion methods 
has its advantages and disadvantages, and as ever it is the job of the engineer 
to balance a multitude of parameters to design a product that represents value 
for money to the consumer.


Еще:

http://www.gaw.ru/html.cgi/txt/doc/dac/index.htm

http://www.yandex.ru/yandsearch?rpt=rad&text=%F1%F5%E5%EC%EE%F2%E5%F5%ED%E8%EA%E0+%F6%E8%F4%F0%EE-%E0%ED%E0%EB%EE%E3%EE%E2%FB%E9+%EF%F0%E5%EE%E1%F0%E0%E7%EE%E2%E0%F2%E5%EB%FC


-- 
Best regards,    Anton Osennikov.
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