The magic of DSD

8/11/2019

The magic of DSD

There are many urban myths surrounding Direct Stream Digital (DSD), so I thought it might be interesting to look at DSD in detail and dispel some untruths. It may be simplified a little, but the following should give an insight into this remarkable format, in the context of Melco, of course, and is also applicable to USB-connected DACs as well as Ethernet streamers.

What is DSD?

Remember bitstream audio? It was used to indicate an elegant digital path in the mastering of LPs before CD was launched. It was a single-bit digital ‘stream’ and was considered a really useful digital format because decoding (D/A conversion) did not depend on high-precision resistive ladders etc. as used in DACs for PCM – at the time, 16-bit DACs were costly and had compromised performance.

PCM requires that the DAC has the ability to differentiate levels approximately 1/65,000 of peak level for 16 bits; more precision required for more bits. This requires an extremely precise ladder DAC or similar technology, whereas DSD is pulse density or delta-sigma modulation of a single bitstream; the decoding requires no special amplitude resolution and is, therefore, easier to implement.

Move the clock forward a few years and SACD uses the same digital stream with good results, making bitstream effectively a consumer format. With the advent of CD – a 16-bit PCM format – DACs have moved on in leaps and bounds since the ‘80s, but still there is the requirement to have extremely precise DAC implementation for 16, 24 or even 32-bit, or to use extreme oversampling/ clever data techniques to get around the basic problems of a PCM ladder DAC.

Bitstream never went away, though, and today it is known as DSD and offers many potential advantages over PCM.

There never was a specific physical carrier (silver disc) for DSD, so the download and digital files environment was necessary to popularise DSD. SACD is the same format but is copy protected, so the files do not move off of the silver disc carrier.

DSD formats

DSD exists in several forms:

There is the concept of single DSD or DSD 64 (being 64 times the sample rate of CD) and this is the original SACD standard. By doubling and quadrupling the data rate we get double DSD, DSD 128 and Quad DSD, DSD 256.

If the data was being created in the studio to master a CD, then the DSD would be 64, 128 etc. times the CD data rate. But if the data is being taken from analogue sources (microphone mixers, tape machines etc.) and is not for CD production, then the convention of 48 kHz sampling equivalent applies.

On the front of a Melco, when playing single DSD, a data rate of 2.8 MHz is shown for CD-related files and 3.1 MHz for 48 kHz related files.

How to justify record company claims ‘from the Master Tape/s’

The sample rate is a really useful clue as to the origins of the expensive download that you might have purchased and shows whether the claim, ‘Direct from the master tapes’ really applies or whether this has been lifted from CD-production data. Direct from an analogue tape will give DSD of 3.1, 6.1, 12.3 MHz etc. But, if the material was lifted from CD production data, the giveaway is the data rate of 2.8 MHz, 5.6 MHz etc.

Markerless DSD

To move DSD from one device to another, it is usually necessary to put the data into packets. This is also the case for USB and Ethernet. The packets contain the DSD data and have additional markers at the start and end of each packet to identify their place in the data queue. These markers take up around 30 per cent of the available bandwidth.

Some recent DACs and streamers can accept data without these markers. Melco supports this and tests the comms automatically. If markerless is possible with the specific DAC then there is a sound quality improvement due to the data rate dropping. There is no requirement for the user to intervene – markerless happens whenever possible; another clever Melco innovation!

DXD

DSD is really convenient but is not possible to easily edit, change levels, fade in and out of tracks etc. This requires PCM.

DXD is a 32-bit, 384 kHz (CD production) or 32/352 kHz PCM format. This is used before the finalised DSD.

Equivalent resolution DSD vs. PCM

The question always asked surrounds DSD vs. PCM resolution. They are totally different digitisation regimes and comparison is not easy, but it helps to think of SACD/single DSD/DSD 64 being something of the order of 20-bit resolution; sample rate equivalence depends on the DAC technique but it’s safe to say 96 kHz.

DSD 128 can be considered equivalent to a 24-bit PCM format.

Playing DSD

There are several DACs that take full advantage of the DSD format. There are a lot more DACs that are based on PCM and, therefore, have to convert the DSD into PCM. Some do it well, some do not, and this is possibly the reason that opinion is divided on the merits of DSD. Conversion from DSD to PCM in the DAC is not so easy, the maths is immense.

With Melco connected to a USB DAC, you can choose! Many DSD-capable DACs are actually PCM DACs with inbuilt conversion. The Melco, however, can very precisely convert DSD into 32-bit PCM. Presenting this PCM data to a DAC designed originally for PCM is probably the best way to use the DAC with DSD.

DSD-over-Ethernet sometimes requires DoP (DSD over Packet) depending on the streamer; there are several one-time options for the Melco to communicate in the most stable way with the streamer.

Buying DSDs or DXD

As mentioned, there is no physical carrier for DSD (other than SACD which is copy-protected). But many vendors offer DSD and (in some cases) DXD downloads. If you purchase the highest resolution files you often can access the lesser files as well, making interesting sound quality comparisons.

Most telling is DXD versus Quad DSD: one uses a PCM DAC and the other a DSD DAC, so it allows a lot of insight into the performance of your hardware.

Listening tips

There are many vendors of DSD, but we do enjoy relationships with HighResAudio.com and Native DSD.com, plus both have samplers available for Melco owners. Particularly fine recordings are: