The I2S input is 150 ohm.
Interesting.... Why does my Master 7 RJ-45 jack connector board have 220 ohm resistors for I2S termination?
Edited by DACLadder - 12/1/13 at 8:01am
Steve's reply for a twisted pair was about 120 ohms imp. For the i2s. Termination so please explain for me . I only gathered info. From both parties and based on what you just stated I do not understand the perimeter. To follow.
I'm going nuts here... Then why does the supplied Off Ramp RJ-45 terminating plug use 75 ohms for the MCLK signal? And why doesn't the first scope capture look great with a 120 ohm resistor? This exactly duplicates the 75 ohm plug but only with a different value resistor. And one more thing to ask Steve.,... If not used do the HDMI I2S outputs need termination for the RJ-45 I2S outputs to function normally - or can HDMI (or any other output) be left unconnected?
Since there is no spec for external I2S it seems one vendor assumes one thing and another something else which doesn't surprise me. I'm just going the have to tune up the I2S interface with what works best for my setup. I'll be sure pass anything significant along.
The resistors is solder on the I2S kit bottom side, you can replace by other resistors, but low impedance request the source have better output drive ability.
Kingwa has some good advice. 75 ohms is quite a large current load for say a normal TTL logic driver. The chip's output impedance would come into play and you may see stair steps on the rising/falling edges. But it appears Off Ramp uses a healthy driver for MCLK at least.
Kingwa also chose a good logic chip for receiving the I2S signals (74VHC125). These chips can tolerate a wide variation of input signal levels without latching up.
I wrote to Steve N. at Empirical. Supplied waveforms. I asked if all the I2S RJ-45 signals are driven the same way as the MCLK signal so if I chose 75 ohms (or something else) could I expect similar performance. I explained what was going on and was just trying to tune up the I2S interface for best performance with the M7.
Here's a scope capture of the Off Ramp MCLK signal with no cable and 75 ohm resistor crimped to a RJ-45 plug. The waveform looks good with a healthy 4V signal and clean edges. Mild overshoot but nothing like 120 ohm or 220 ohm termination. I would be very happy with this signal quality at the M7 end point.
Let's see what Steve N. at Off Ramp has to say.
Steve N. from Off Ramp maker responded and said it is OK to use 75 ohm termination on all I2S signals. Each of the four Off Ramp I2S signals are driven the same way so nothing special about MCLK.
Steve recommended termination directly at the endpoint chip per transmission line theory. The RJ-45 jack connector board, where termination is currently located, is not the end of the signal path as there is still a section of ribbon cable to go to reach the I2S receiver chip on the M7 DSP board. Adding termination at the very end point chip would be very difficult to do for the casual electronic enthusiast especially with no schematic or guidelines. If termination at the M7 I2S RJ-45 connector board doesn't work out then building a special ribbon cable with terminations at the DSP header end is the next best option (remove the RJ-45 jack board terminations first).
But first will take the easy road and change the RJ-45 jack connector board terminations from 220 ohm to 75 ohms and see how well the signals look with the oscilloscope. At the very least I believe they will look better than they do currently. Then evaluate and go forward to something more exotic if needed. 75 ohm resistors on order so standby.....
Boy, I'm getting tired of writing up tests and I'm sure you are tired of me as well hogging all the ink. But I have now have different termination on the four RJ-45 board termination locations. I have a bunch of 120 ohm resistors and by luck 120 ohms in parallel with 220 ohms yields about 77 ohms. 77 ohms close enough for testing. The waveforms look a LOT better but not perfect. So let me listen to this for a while and get back in detail next week.
But I am now finding less variation sound wise with external I2S cables. The short CAT 7 cable sounds very much like the 18" CAT 5. Previously when I2S didn't sound quite right bass is weaker and the highs slightly fuzzy. One day sound great and the next a little different. Also for the past few days with the Master 7 on the workbench been listening to ADG Ref. 5.32 and USB32. Didn't sound bad at all with very warm sound in comparison. Now the Off Ramp 5 and Master 7 I2S are tuned up somewhat they kick the Ref 5.32 in the rear end. The OR5/M7 sound is vibrant and clear - crystal highs, tight bass, and great vocals.
Again, I'll be listening to the new RJ-45 board terminations for a while to fully see how it works out long term. And if anyone has a better approach I am all ears...