Direct Stream Digital and the Super Audio CD: A Revolution in Digital Audio

1. Direct Stream Digital and the Super Audio CD: A Revolution in Digital Audio

2. What�s wrong with my CD? The standard Compact Disc currently in use has a frequency response of 0 to 20 kHz and dynamic range of 96 dB. While humans can only detect tones up to 20 kHz, higher frequencies have been shown to affect timbre of perceived sound. Human hearing has a dynamic range of about 140 dB.

3. What is our current situation? Nearly all digital audio in the past has used a scheme called �Pulse Code Modulation� or PCM. PCM represents an audio waveform as points on a grid. Sampling rate determines the interval between data points on the time axis. Bit resolution determines the interval between data points on the amplitude axis.

4. PCM represents audio as points on a grid.

5. What is Direct Stream Digital? Direct Stream Digital (DSD) uses a completely different scheme. Audio waveform is represented in a pulse train. The density of pulses determines the height of the waveform.

6. DSD represents audio as a pulse train. DSD Pulse train example picture DSD Pulse train example picture

7. How does it work?

8. So what�s that Delta-Sigma thing?

9. Another way to look at it�Matlab Code function modSig=deltaSigma(sig) % Input is any signal 'sig' % Function outputs delta sigma modulated version of input % Normalize the input signal so that its maximum amplitude is 1 % (only scales down, not up) if (max(sig) > 1) normSig=(sig./(max(sig))); else normSig=sig; end % Set the initial value for the modulator if (normSig(1)>=0) mod=1; else mod=-1; end % Set the initial value for the sum sigma=0; % Create an empty array for the output signal modSig=[];

10. Let�s give it a try�

11. Another example�

12. And one more�

13. Sample outputs�

16. Getting around. PCM can be converted to DSD by means of a delta-sigma modulator. This time the input to the modulator is digital instead of analog. DSD can be converted to PCM by means of an FIR filter. This is a non-recursive filter that basically averages out the pulse train and then quantizes to the desired number of bits.

17. Is DSD, in fact, better? Lets look at the specs�

18. PCM can�t deliver it�s promised frequency response because it must use low pass filters to prevent aliasing, a type of distortion. Not only that, but the steep filters used to prevent aliasing often cause time alignment problems in the high frequencies. While the 144 dB dynamic range of 24 bit PCM is quite impressive, it is probably unnecessary. But wait, that�s not all�

19. Further more� DSD can only deliver its promised 120 dB of dynamic range in the 0 � 20 kHz range. Since it uses noise shaping to achieve such a high dynamic range, it sacrifices dynamic range in the upper, inaudible, frequencies for improved dynamic range in the lower, audible, frequencies.

20. A quick look at noise shaping�

21. Things get hairy. (Even more complications to the issue) Low cost PCM A/D converters are already using delta-sigma modulators, because they are inexpensive and easy to implement. High quality DSD converters use multi-bit delta-sigma modulators because they are free from certain types of distortion. The signal is later converted to the 1-bit stream.

22. All current digital signal processing operations require a multi-bit signal. DSD workstations must convert to multi-bit for processing, then back to 1-bit for final delivery. Many audio professionals wonder whether a high sampling rate, 4 or 5 bit, system using delta-sigma modulation might be a better solution.

23. The Super Bit Mapping Direct technology from Sony allows a high precision down-conversion from DSD to CD quality PCM. The result is better sounding CDs.

24. DSD on Disc, the Super Audio CD Single sided, double layer disc. One layer contains the standard CD version of the disc, on the other is the SACD version. Due to loss-less data compression, the SACD layer can hold both a Stereo and 6- channel version on the same disc!

25. More goodies� The SACD layer can also hold text, graphics, and video. Completely backwards compatible with standard CDs. 74 minute playing time. (Same as CD)

26. Copy Protection On a standard CD, digital information is represented as small pits of varying length. On a Super Audio CD, the widths of pits are varied as well in order to implement a digital watermark. Requires special optical sensor, won�t play on existing DVD-ROM drives. Discs without the special watermark won�t play. Very difficult to pirate.

27. The Contender� DVD Audio (DVD-A) can hold 2 to 6 channels of PCM audio. Quality can range from 44.1 kHz/16 bit to 192 kHz/24bit. Playing time varies depending on quality and channels. More difficult to author than the SACD

28. Politics DSD/SACD is backed by Sony and Philips, two huge companies with lots of marketing clout. These are the same companies that developed the original CD. The DVD-A is backed by a conglomeration of companies who often do not get along.

29. Conclusions 99% of people will probably not be able to tell the difference between high resolution PCM and DSD. DSD/SACD will probably win the marketing war. DSD/SACD is the most effective means available to deliver digital audio to the consumer. DSD is not the most effective solution for professional recording and processing, although it is a large step above other schemes in use.

30. DSD/SACD will be much more beneficial to very dynamic and acoustic music (such as Classical and Jazz) than to pop and electronic music. Only some people will have good enough equipment and ears to hear the difference between a standard CD and a SACD. It will take the SACD a long time to reach the mainstream market if ever.