Talk:APU DMC

Similarly to the noise channel, the DPCM channel's frequency counter on the die is a 9-bit linear feedback shift register (with taps at the 5th and 9th bits); when I take the counter values from the on-die ROM and run the LFSR until the result is '100000000', the cycle counts (for NTSC) match once multiplied by 2. --Quietust 05:00, 23 January 2011 (UTC)

I suspect there's no "address increment", but another shift register. Just pay attention to the address wrap - if we had a counter, it would wrap to zero, but instead, it wraps to $8000, like ($10000 >> 1). --Zepper 01:49, 6 June 2011 (UTC)

The generated addresses are linear, and I don't think a shift register can do that. So I'm conjecturing an up-counter here. --Tepples 02:30, 6 June 2011 (UTC)

The DPCM address register is only 15 bits wide (with A15 being connected to the VCC rail @ 2850,5450), and the upper bit (A14) gets initialized to 1 (@ 2565,5365) instead of the last value written to $4012 (and the bottom 6 bits get initialized to 0, but we already know this). --Quietust 03:03, 6 June 2011 (UTC)

DMC find

Looks like the DMC silent flag makes difference for $4011 raw output. If this flag is non-zero, the $4011 value written is ignored by the raw output only. I could test a few NSFs and it works flawlessly. --Zepper 14:53, 14 November 2011 (UTC)

By "the DMC silent flag" do you mean bit 4 of $4015? And what exactly do you mean by "ignored by the raw output only"? Do you mean that values are queued up and take effect after the sample period ends? --Tepples 15:10, 14 November 2011 (UTC)

Well, the silent flag is described in the APU DMC section, "The output unit continuously outputs a 7-bit value to the mixer. It contains an 8-bit right shift register, a bits-remaining counter, a 7-bit delta-counter, and a silence flag". Just read the rest of the APU DMC output unit. About the raw output, I mean the timed writes to $4011 register, as listen in Battletoads (drums) for example. --Zepper 15:31, 14 November 2011 (UTC)