PCM (I2S) and DSD on the Same Wires

DACs SHARING INPUT PINS

DACs using same input pins for handling PCM and DSD are not new but are not very common. If we peruse the web, we find a few of them. Mr. Bunpei has listed a few of them in this post: [981] many months ago.

• The Sabre32 ESS9018 DAC is of course one of them (through the use of clever wiring)
• The TI PCM 1795 (page 21 of datasheet [link]) and the AKM 4399 (page 4 of datasheet [link]) are two other DACs that share the same pins for PCM and DSD.

Most DACs however, have separate pins for DSD and PCM. For example two well known DACs, the DSD1794 and WM8741 have separate connections for PCM inputs and DSD inputs.

DACs sharing input pins for DSD and PCM are interesting because of the potential of switching between DSD and PCM without the need of switching input wires, and if used with an interface that can equally transmit both PCM and DSD through the same output pins, then there is no need for using any external switching mechanism that can affect the signal integrity so preciously sought after by audiophiles 🙂

AUDIO INTERFACES SHARING OUTPUT PINS

Audio interfaces sharing the same output pins for DSD and PCM seems a new thing. So far, I am only able to identify these devices:

• Amanero USB interface
• SDTrans memory player [link]
• XMOS USB Audio 2.0 Reference Design Board [link]. Firmware here [link], [link]. Other XMOS-based interfaces may also support DSD/PCM with the aforementioned firmware.

These interfaces, together with DACs that share input puts for DSD and PCM  allow the “transparent” and even automatic switching of DSD and PCM without the need for external signal switchers. This is the appealing part for audiophiles. The “less is more” approach; the “direct wire” connection… 🙂

Unfortunately as of this writing, two highly regarded interfaces, the XMOS WAVE IO [link] and the EXAU21 [link] do not support this “mode” of sharing output pins (WAVE IO does not support passing DSD yet, but it is just a matter of firmware). These interfaces would require an external device to switch between PCM and DSD outputs.

PIN MAPPING

Using the same pins to pass PCM an DSD is not enough to enable the passing of the signals and the “transparent” and automatic switching between PCM and DSD. For this to happen, the  pin assignment of the audio interface must be compatible with the pin assignment of the DAC.

Here is a table of all the compatible DACs and interfaces showing how the DSD pins are mapped to the PCM/I2S pins

I2S Pins	ESS9018 [link]	PCM1795 [link]	AKM4399 [link]	Amanero(1) [link]	SDTrans [link]	XMOS Ref [link]
BCLK	DSD Clock	DSD Clock	DSD Clock	DSD Clock	DSD Clock	DSD Clock
LRCLK	DATA Left	DATA Right	DATA Right	DATA Left	DATA Left	DATA Left
DATA	DATA Right	Data Left	Data Left	Data Right	DATA Right	DATA Right

Notes:

(1) With driver 1.0.56, If using driver 1.0.5 or earlier, then the channels are swapped.

As shown above, there doesn’t seem to be a standard way to map the DSD DATA pins to the I2S LRCK an DATA pins. It seems SDTrans, XMOS Ref and Amanero with the latest driver are matched with the ESS 9018 DAC channel mappings (for good reason).

If you wish to use the interfaces with the other DACs, then the channels need to be reversed. Fortunately, they can be remapped in foobar as shown here [link]

(Update 11/22/12): Amanero has released a new version of the firmware [link] reversing the channel mapping. (Although the driver with the older firmware fixes the problem). However, there are reports of some bugs, so wait until a more stable version or more testing is done before upgrading the firmware. In the meantime if you still have the old firmware, just use the new driver and enjoy experimenting 🙂