Buffalo2-Sabre32 DAC MEGA Test

MOTIVATION

I Decided to figure out how the Sabre32 works in the Buffalo II implementation with respect to quantization bit depth, input mapping, polarity and true/pseudo balanced mode. The Sabre32 data sheet gives NO information in this area, but these topics have been discussed to some degree by the designers at diyaudio for the benefit of us audio tweakers. My starting point was post 35 by Dustin Forman in the monster ESS Sabre Reference DAC thread.

Although the information contained in that thread seems complete, it is not easy to understand. The ensuing discussion on the topic by different members of the forum left questions in my mind. Therefore, in order to get a better understanding the behavior of the DAC I had  to test it myself.

I wrote a huge case statement and used the rotary encoder to move from one test to another [another advantage of having a rotary encoder]. Here are the results.

For reference these are the Sabre32 Default register settings at power-on:

• Quantizer: 6-bit
• DAC Phase: In-Phase
• DACB Phase: Anti-Phase
• Differential: True
• Input Map: Normal

TEST RESULTS

Input mode tested: I2S and SPDIF. Results apply to both unless noted in the results column

#	Q	DACB Phase	Diff	Input Map	Results
1	6-bit	Anti-Phase	True	Normal	I2S:Sound is ~6dB lower –> Half of the DACs are not used; SPDIF: Normal sound. In SPDIF mode, the DAC is automatically set to STEREO mode, utilizing all the DACs
2	6-bit	Anti-Phase	True	B-II	Normal sound. In I2S, BII wiring corresponds to the input channel Remap, thus all the DACs are used
3	6-bit	In-Phase	True	Normal	No sound –> True diff cancels out sound as expected
4	6-bit	In-Phase	True	B-II	No sound –> Same as above
5	7-bit	Anti-Phase	Pseudo	Normal	I2S: Sound is ~6dB lower –> Half of the DACs are not used because they do not have input signals; SPDIF: Normal sound. In SPDIF mode, the DAC is automatically set to STEREO mode, utilizing all the DACs
6	7-bit	Anti-Phase	Pseudo	B-II	Normal sound as expected
7	7-bit	Anti-Phase	True	B-II	Normal sound –> True differential is also supported in 7-bit mode for both SPDIF and I2S
8	7-bit	In-Phase	Pseudo	Normal	I2S: Distorted sound, ~6dB lower –> Half of the DACs are not used and Pseudo diff does not cancels out sound; SPDIF: Distorted sound (heavier because all DACs are used). Not expected as DACBx being in-phase should cancel-out the sound of DACAx which is also in-phase
9	7-bit	In-Phase	Pseudo	B-II	Distorted sound, normal level –> All channels are used in both I2S and SPDIF. Pseudo diff seems not an exact anti-phase of the signal as there is no full cancellation. This is not expected behaviour
10	7-bit	In-Phase	True	B-II	No sound –> exact anti-phase cancels out sound as expected
11	7-bit	Half-Half-1	Pseudo True	B-II	I2S: Distorted, but less distortion than #9 –> Half the DACs anti-phase contributes to normal sound, half of the DACs in-phase contribute to distortion. SPDIF: Distorted sound full volume as all the DACs are used.
12	7-bit	Half-Half-2	Pseudo True	B-II	I2S: Distorted, but less distortion than #9 –> Half the DACs anti-phase contributes to normal sound, half of the DACs in-phase contribute to distortion. Should expect normal sound if using normal input map (In normal map, the DACs contributing to the distorted sound will not be connected). See test #28. SPDIF: Distorted sound full volume as all the DACs are used
13	8-bit	Anti-Phase	Pseudo	Normal	I2S: Normal sound –> In 8-bit, only half the DACs are used according to post 35 even though BII wiring only feeds half of the input channels; SPDIF: Normal sound as expected
14	8-bit	Anti-Phase	Pseudo	B-II	I2S: Normal sound –> In 8-bit. This test and test 13 shows that it does not matter whether in Normal or B-II input mapping; SPDIF: Normal sound
15	8-bit	Anti-Phase	True	B-II	Normal sound –> True differential is also supported in 8-bit mode for both SPDIF and I2S
16	8-bit	In-Phase	Pseudo	Normal	Distorted sound –> All channels are used in both I2S and SPDIF. Pseudo diff seems not an exact anti-phase of the signal as there is no full cancellation
17	8-bit	In-Phase	Pseudo	B-II	Distorted sound –> All channels are used in both I2S and SPDIF. Pseudo diff seems not an exact anti-phase of the signal as there is no full cancellation. As found in other tests, in 8bit mode the input mapping does not matter
18	8-bit	In-Phase	True	B-II	No sound –> As expected. In true differential, the anti-phase signal is the exact 180 degree signal
19	8-bit	Half-Half-1	Pseudo	B-II	Distorted sound
20	8-bit	Half-Half-2	Pseudo	B-II	Distorted sound
21	9-bit	Anti-Phase	Pseudo	Normal	Normal sound
22	9-bit	Anti-Phase	Pseudo	B-II	Normal sound
23	9-bit	Anti-Phase	True	B-II	Distorted sound. True differential is NOT supported when 9bit is used. In I2S I noticed increased volume of distortion. I don’t know what this means. I did not compare distortion levels when using SPDIF input
24	9-bit	In-Phase	Pseudo	Normal	Distorted sound
25	9-bit	In-Phase	Pseudo	B-II	Distorted sound
26	9-bit	Half-Half-1	Pseudo	B-II	Distorted, In I2S I noticed the least amount of distortion. I don’t know what this implies. I did not compare distortion levels when using SPDIF input
27	9-bit	Half-Half-2	Pseudo	B-II	Distorted sound
					Additional Tests
28	7-bit	Half-Half-2	Pseudo	Normal	I2S: Normal Sound, ~6 dB lower volume. –> Indicates that DAC channels 3,4,7,8 are not being used because they are in-phase pseudo differential and as such (if used) they should distort the sound according to tests 11 and 12; SPDIF: distorted, since all channels are used in 7-bit regardless of input mapping
29	9-bit	in-Phase	True	B-II	No sound. Although this seems to be the correct behavior, it does not agree with test #23, where the anti-phase signal results in distorted sound in true differential mode

The following tests (30-37) are designed to test one channel at a time with true differential mode. True differential works For 7bit and 8bit as tests 7 and 15 show for anti-phase and tests 10 and 18 show for in-phase. Now, selecting a single channel anti-phase with the rest in-phase for DACBx should result in normal sound (at lower volume level since only one DAC out of eight does not cancel out).

However,  the results below show that except for 6-bit, the sound is distorted. I cannot explain why the sound is distorted…

Input: I2S. for SPDIF I only tested 7bit, normal input mapping

#	Q	DACB Phase	Diff	Input Map	Results
30	6,7 8,9	in-Phase; ch1 anti	True	Norm B-II	Sound in LEFT channel only: 6bit: normal; 7,8,9bit: Distorted
31	6,7 8,9	in-Phase; ch2 anti	True	Norm B-II	Sound in RIGHT channel only: 6bit: normal; 7,8,9bit: Distorted
30	6,7 8,9	in-Phase; ch3 anti	True	Norm B-II	Sound in LEFT channel only: 6bit: normal; 7,8,9bit: Distorted. 8bit: also distorted but volume level is lower than ch 1,2,7,8
33	6,7 8,9	in-Phase; ch4 anti	True	Norm B-II	Sound in RIGHT channel only: 6bit: normal; 7,8,9bit: Distorted. 8bit: also distorted but volume level is lower than ch 1,2,7,8
34	6,7 8,9	in-Phase; ch5 anti	True	Norm B-II	Sound in LEFT channel only: 6bit: normal; 7,8,9bit: Distorted. 8bit: also distorted but volume level is lower than ch 1,2,7,8
35	6,7 8,9	in-Phase; ch6 anti	True	Norm B-II	Sound in RIGHT channel only: 6bit: normal; 7,8,9bit: Distorted. 8bit: also distorted but volume level is lower than ch 1,2,7,8
36	6,7 8,9	in-Phase; ch7 anti	True	Norm B-II	Sound in LEFT channel only: 6bit: normal; 7,8,9bit: Distorted
37	6,7 8,9	in-Phase; ch8 anti	True	Norm B-II	Sound in RIGHT channel only: 6bit: normal; 7,8,9bit: Distorted

TEST NOTES

1. When the sound is distorted, it is heavily distorted. Like a radio station not quite tuned into the correct frequency
2. In all tests DAC Phase is In-Phase (Each channel consists of a DACx and DACBx pair)
3. Normal input map: each channel maps to a separate input put (each pin requires input signal)
4. B-II input map: input pins are shared by two channels as specified here
5. Half-Half-1: DACB 1,2,5,6 In-Phase; DACB 3,4,7,8 Anti-Phase
6. Half-Half-2: DACB 1,2,5,6 Anti-Phase; DACB 3,4,7,8 In-Phase
7. All tests conducted with Buffalo II DAC without output stage. Balanced output feeds directly a UCD180HG balanced amplifier

CONCLUSION

From a theoretical point of view, there are still questions… From a practical point of view though, there are 6 configurations that are supported by the Buffalo II implementation of the Sabre32 DAC (that is, using the input remap to match the pin wiring):

1. 6bit, true differential
2. 7bit, pseudo differential
3. 7bit, true differential
4. 8bit pseudo differential
5. 8bit true differential
6. 9bit pseudo differential

I plan on adding these 6 selections on the next revision of the software.

REFERENCE

Sabre8 GUI application: [link]. This application also includes a help file where you can find out information about the register settings that is not included in the data sheet.