Originally Posted by phwatts
- What is the purpose of the additional BNC plug you installed in the place of the composite video socket? Is it just a superior S/PDIF input from the source without an input mux?
In addition to the AES output, I also have a buffered output of the VCA SPI buss on an 8pin mini DIN connector. I put that in place of the second video output RCA and since the OEM jack was a dual RCA, I just stuck a BNC in the hole for the video out which I just use for the menu. The SPI jack was for a previous application.
- You mention internal fs of 96kHz, so to confirm it means the inputs are forced to 96kHz for a unified processing rate and thus the DAC's also always run at 96kHz?
Yes the MC4/8/12 runs 96khz as its native rate.
- What is the frequency multiplier of the MCK to the DAC/DIT? I would presume it's 24.576MHz as the commonly used 256xfs?
Yes IIRC, it's 24.576
- How does the MC8 volume control work? If you say the control is disabled does I guess volume control is not done in the DSP. Does it therefore mean they use a) programmable gain amplifiers like PGA2310 as used in e.g. Proceed or b) digital attenuation in the DAC?
The MC 4/8/12 uses analog gain control via BB PGA2310 VCA chips. (Actually they are switched resistance ladders and not true VCA's). So the volume control on the MC8 does not work on the AES output.
- How are other volume offsets like speaker level calibration of the various channels treated? Is this therefore also done in your outboard unit and not used in the MC8?
I have a 24 channel analog volume contol box after my DSP farm and Lucid DACs. This has 24 channels of PGA2310's and it is here where I do the speaker trims.
- Could you please elaborate on how Lexicon uses the VCA's to produce the "mock surround modes"?
This is speculation as surround modes are Lexicon's secret sauce but it is believed that some channel gain tweaking is used in the synthesized surround mode. I even built an interface from that SPI buss connector to a PC parallel; port so I could analyze what they are doing, if anything, to the gains for various surround modes. But I since determined it's not worth the effort to try and implement this function. I am happy with the surround modes as is.
- Are the AK4395 DAC's used in hardware ("parallel") or software control modes, and if in software, is any dynamic/run-time control done or is it just an initial config?
- Likewise, I presume your DIT is just run in hardware mode without a MCU or hacking the MC8 to configure its SPI port?
IIRC, they run in software config mode. I don't alter the MC8 SPI. I just tap the I2S at the DACs, run through an FPGA which in retrospect is not needed (I put it there just in case but have it "wired through") then onto to an AKM4104 quad AES transmitter chip.