Hi. My name is Peter Scheelke. I am one of the founders of DAD, Digital Audio Denmark. We are the manufacturer of the
AX24 AD/DA converter and the Merging Sphynx 2 converter.
In the discussion about DSD/DXD,
Sadie/Sonoma/
PYRAMIX I can help clarify some of the advantages/disadvantages in the different audio formats.
DSD 64fs (the format used for SACD) is a 1 bit format with a sample rate of 2.8224 MHz (64 x 44.1 kHz). The main advantage of this DSD format is the as close to perfect as we have ever seen impulse response, which is just a little more than 100% measured with a 3us pulse, plus the very wide frequency band. The disadvantage of DSD compared to digital PCM is the noise performance and the fact that DSD can not be edited since it is only 1 bit.
In our
AX24/Sphynx 2 we have been able to keep the noise down until 23 kHz (<-120 dBfs measured 20-20 kHz), however the energy from the noise is still part of the signal, so after 23 kHz the noise becomes increasingly significant.
Since DSD can not be edited as a one bit signal, it is necessary to convert to some kind of multi bit for editing. When converting back to DSD the modulator noise will be added again, and after just a few editing back and forth the audio will no longer comply with the noise performance specification stated in the Scarlet Book (specifications for SACD’s). DSD 64fs is used as recording format in Sonoma,
Sadie and
PYRAMIX DSD systems. (
PYRAMIX can also record in DXD).
There is also DSD 128fs, also 1 bit, but with a sample frequency of 5.6448 MHz (128 x 44.1 KHz). Since it is a 1 bit signal, quantization noise is equal to DSD64, but since the frequency band is twice as wide, the noise is only half that of DSD 64fs within a given frequency band. With the same type of noise shaping as DSD 64 the noise will start rising at the double frequency. In our implementation the noise starts rising from about 46 kHz.
PCM 44.1-192 kHz/24 bit: Since all frequencies, at more than half the sample rate, will be mirrored around half the sample rate (aliasing), all PCM formats needs an anti aliasing filter. The normal anti aliasing filter is the 0.45/0.55 filter which starts at 45% of the sampling rate and has full attenuation at 55% of the sampling rate.
A major disadvantage in the normal 0.45/0.55 anti aliasing filter is that the filter is only attenuating 10-12 dB at half the sample rate (Nyquist), so frequencies between 50% and 55% of the sample rate will get mirrored around half the sample rate and will create new frequencies without any harmonic relationship to the audio. Another disadvantage is that some of the energy from the audio is lost in pre/post ringing; a stronger anti aliasing filter will create more pre/post ringing than a less intense filter. Since some of the energy is lost, the anti aliasing filter attenuates the impulse response.
Due to bandwidth a steep anti aliasing filter at 44.1 and 48 kHz sampling rate can be justified, however at higher sampling rates (96kHz, 192kHz) it would be better to use a less steep filter. All anti aliasing filters cause delay in the A/D converter which is about 0.8 ms at 44.1 kHz sampling rate with a 0.45/0.55 filter.
DXD is something new, a 352.8 KHz/24 bit sampling rate in the A/D converter intended for 32 bit floating point DAWs. Until now DXD is supported by DAD, by Merging in their
PYRAMIX DAW, by AMS Neve in their new DFC PS/1 console, and in the
Saracon SRC from Daniel Weiss, but also other manufactures are one their way with DXD equipment.
In our implementation we have chosen to add only very soft anti aliasing filter for DXD and for an even higher sample rate we offer, 384 kHz; since there is very little audio above half of the sample rate. Our implementation of DXD has a great impulse response (88% with a 3us pulse) and a significant better out of band noise performance compared to DSD64fs.
We think it’s a big step forward, offering far better impulse response that previous PCM, avoiding the filtering problems of previous PCM and better out of band noise performance than DSD. We think this is the future of digital audio.
Merry Christmas from Denmark to all audio lovers at Gearslutz!
Peter