5 ms·

Well, I haven't done audio programming for quite a while. I did get audio hardware behave pretty nicely in 1998, though. I believe low latency audio was much ha

by vardump 10y ago

Well, I haven't done audio programming for quite a while. I did get audio hardware behave pretty nicely in 1998, though. I believe low latency audio was much harder back then. I'm pretty surprised if your deadline is really that 23 microseconds (60 fps / 7000). You can't even reliably dispatch an IRQ request in that time! > I wouldn't call that similar If you have a long audio filter chain it's not all that different from a long fragment shader. You can think of a fragment shader as a filter chain. > Since you can't provide any advise for an alternative to function pointers Uh. That's pretty hard without seeing the source code and full problem description. Especially because you seem to be expecting some magical solution. Well, those don't exist, it's just hard work. Besides, like I said earlier, sometimes function pointers are appropriate for the task. Just understand the cost. Do keep in mind audio filter chains can be pretty complicated. If your filter chain has many concurrent channels with hundreds of dynamic filter steps for each sample to pass through, yeah, you might have some performance problems. > i must challenge your expertise in audio programming since you seem to project your view from a graphics programming point. Be my guest. Just wanted to share something I know about and give new ideas. Btw, I'm not doing graphics, but mostly I/O with micro/millisecond level deadlines.

iammyIP 10y ago
Getting audio hardware to behave nicely for audio is pretty nice :) The calculation for the time factor difference is simply 44100/60 which is about 733, i don't know how the 23 microseconds you got there relate to this. Your comparision to graphics seem to relate to long FIR filters, which i dont't use because of cache reasons (big kernel/lookup table). FIR filters can however sound pretty nice with static parameters but get extremely expensive for realtime modulation. For realtime modulation i suggest IIR filters (Finite Impulse Response vs Infinite Impulse Response filter architecture). Of course it's hard to give advice into the blue without any source, but i atleast expected some general hint, like 'avoid branches, use function pointers' on a next-level, since you were so opposed to function pointers to begin with. I am also not completely sure what's the best approach in this regard, and i surely did not enough profiling in this regard. That said, i remain sceptical regarding your critique of function pointers in a single threaded audio application, because of a lack of an alternaltive. I/O on a microsecond critical deadline certainly seems to be in a neigbourhood to these problems. Do you use some arcane javascript asm.js with some jit auto-reconfiguring optimising for this, or how would you describe your approach?
- vardump 10y ago
  > i don't know how the 23 microseconds you got there relate to this 1 / (60 Hz * 700) = 23.8 microseconds. (Yeah, should have rounded up to 24). What worked for me in the past was just concatenating (well, memcpying) executable filter blocks of x86 code while ensuring parameters were in the right registers. Loop start block -> filter 1 -> filter 2 -> filter 3 ... -> loop end block. I made these blocks by processing specifically compiled (not at runtime, but ahead of time) functions, stripping prologues and epilogues. There's more to it, but it's been a while. Crude, but effective and fast even when each filter step was pretty small on its own.
  - iammyIP 10y ago
    So you put in the parameters into registers 'per hand' in assembly and then memcopyed some function to the right adress (using a function pointer?) to process these registers? Wow. That sounds pretty complicated. For now i just use gcc and function pointers and let gcc do the rest. About the 1/(60 * 700) what does that number even mean to you? 1/60 is the usual refresh rate in seconds, about 16 ms, why do you multiply this with 700, this makes no sense?
    - vardump 10y ago
      You said earlier: > Graphics in a minimum 60 fps context is about 700 times less time sensitive. 16.6 ms (60 fps) / 700 = ~24 microseconds. > So you put in the parameters into registers 'per hand' in assembly and then memcopyed some function to the right adress (using a function pointer?) to process these registers? Wow. That sounds pretty complicated. For now i just use gcc and function pointers and let gcc do the rest. I concatenated blocks of compiled code together (compiled as position independent), avoiding branches and indirect calls. Loop setup block took care of putting right parameters to right registers. Hardest part was getting the filter parameter passing right. Dynamic configuration was trivial after that. There were no function pointers except one to trigger the top level loop.
      - iammyIP 10y ago

by vardump 10y ago

iammyIP 10y ago

Getting audio hardware to behave nicely for audio is pretty nice :) The calculation for the time factor difference is simply 44100/60 which is about 733, i don't know how the 23 microseconds you got there relate to this. Your comparision to graphics seem to relate to long FIR filters, which i dont't use because of cache reasons (big kernel/lookup table). FIR filters can however sound pretty nice with static parameters but get extremely expensive for realtime modulation. For realtime modulation i suggest IIR filters (Finite Impulse Response vs Infinite Impulse Response filter architecture). Of course it's hard to give advice into the blue without any source, but i atleast expected some general hint, like 'avoid branches, use function pointers' on a next-level, since you were so opposed to function pointers to begin with. I am also not completely sure what's the best approach in this regard, and i surely did not enough profiling in this regard. That said, i remain sceptical regarding your critique of function pointers in a single threaded audio application, because of a lack of an alternaltive. I/O on a microsecond critical deadline certainly seems to be in a neigbourhood to these problems. Do you use some arcane javascript asm.js with some jit auto-reconfiguring optimising for this, or how would you describe your approach?

vardump 10y ago
> i don't know how the 23 microseconds you got there relate to this 1 / (60 Hz * 700) = 23.8 microseconds. (Yeah, should have rounded up to 24). What worked for me in the past was just concatenating (well, memcpying) executable filter blocks of x86 code while ensuring parameters were in the right registers. Loop start block -> filter 1 -> filter 2 -> filter 3 ... -> loop end block. I made these blocks by processing specifically compiled (not at runtime, but ahead of time) functions, stripping prologues and epilogues. There's more to it, but it's been a while. Crude, but effective and fast even when each filter step was pretty small on its own.
- iammyIP 10y ago
  So you put in the parameters into registers 'per hand' in assembly and then memcopyed some function to the right adress (using a function pointer?) to process these registers? Wow. That sounds pretty complicated. For now i just use gcc and function pointers and let gcc do the rest. About the 1/(60 * 700) what does that number even mean to you? 1/60 is the usual refresh rate in seconds, about 16 ms, why do you multiply this with 700, this makes no sense?
  - vardump 10y ago
    You said earlier: > Graphics in a minimum 60 fps context is about 700 times less time sensitive. 16.6 ms (60 fps) / 700 = ~24 microseconds. > So you put in the parameters into registers 'per hand' in assembly and then memcopyed some function to the right adress (using a function pointer?) to process these registers? Wow. That sounds pretty complicated. For now i just use gcc and function pointers and let gcc do the rest. I concatenated blocks of compiled code together (compiled as position independent), avoiding branches and indirect calls. Loop setup block took care of putting right parameters to right registers. Hardest part was getting the filter parameter passing right. Dynamic configuration was trivial after that. There were no function pointers except one to trigger the top level loop.
    - iammyIP 10y ago