Re: [LAD] hard realtime performance synth

On Thursday 28 January 2010, at 21.01.38, David McClanahan <david.mcclanahan(a)gmail.com> wrote: [...] This has nothing to do with the definition of hard realtime. These lowlatency kernels don't even claim to be *hard* realtime. I think you'd get "lowlatency" or possibly "firm realtime", depending on who you ask. True hard realtime systems don't really exist, but if we accept hardware failure and application bugs as acceptable reasons for failure, we can get pretty close. For Linux, you'd go with RTAI or RT-Linux. However, although running all realtime audio code under RTAI or RT-Linux might offer a slightly lower risk of drop-outs, those are pretty harsh environments, even compared to the "strict" requirements of JACK clients, realtime safe LADSPA or LV2 plugins and the like. You basically cannot use ANY syscalls, but have to work with a completely different API. (RTAI/LXRT does allow hard realtime code to run in userspace, but when a thread is actually running in hard realtime context, it will be thrown back to "standard" mode as soon as it makes an ordinary syscall.) More seriously, you cannot make use of ANY normal Linux drivers. Drivers have to be ported, so that all code involved in the hard realtime "loop" actually runs under the realtime kernel, and not Linux. Even more seriously, there is just no way that any realtime kernel can ensure anything on bad hardware. If BIOS super-NMIs are blocking normal IRQs every now and then, or you have some device + driver combo that abuses PCI bus blocking (common issue with 3D accelerators), you may get worst case latencies of any number of milliseconds - and there is nothing at all that any OS can do about this. You *may* be able to avoid these issues by replacing cards, patching or reconfiguring drivers or tweaking the BIOS settings, but as general purpose PC/workstation hardware isn't really designed for this sort of work, there are no guarantees. It basically comes down to trying hardware until you find something that does the job. BTW, this has very little to do with raw performance. A faster CPU does let you finish the job quicker, but if you wake up too late, you still won't be able to make the deadline... The bottom line is that, in the context of mainstream hardware and systems that aren't specifically put together for lowlatency audio work, this is a matter of diminishing returns. Indeed, it's possible to do better than a lowlatency kernel, but it's a lot of work, and it's completely wasted without perfectly configured, well behaved hardware. Sure, RTAI or RT-Linux would support 0.1 ms audio latency on a purpose built system with ported drivers, properly configured BIOS, SMI hacks etc, but it just won't happen on your average PC. Well, yes - but going beyond lowlatency kernels, it's usually the hardware you need to deal with; not the OS... Same difference. If you have sufficiently solid scheduling for the realtime processing part, you can build pretty much anything around that. [...] It's not about sophistication. A low cost singleboard computer with an AMD Geode, VIA C7, some Intel Celeron or whatever you need in terms of raw power, will do just fine - as long as the chipset, BIOS and connected devices are well behaved and properly configured. If you, as a manufacturer of synths or similar devices, don't want to try a bunch of different motherboards for every new revision you make, you might decide to design your own board instead. Then again, if your product is low volume and requires enormous CPU power, carefully selected mainstream hardware may still be a better option. SMI is one of them. In my experience, nearly every motherboard at least has some BIOS features you must stay away from, so even "know good" hardware sometimes need special tuning for this sort of work. General purpose computers just aren't built for low latency realtime work - but most of them can still do it pretty well, with some tweaking. [...] [...] That's just pointless, as the ADC and DAC latencies are already several sample periods, and the way DMA works on any PCI, USB or 1394 soundcard will add somewhere around 64 bytes' worth of latency or more to that. Also note that your average MIDI synth has anywhere from a few through several tens of milliseconds of latency! You can only send around 1000 messages per second over a standard MIDI wire anyway, so where would you get the timing information to make use of less than 1 ms latency? Actually, going below a few ms only guarantees that the notes in a chord can never be triggered simultaneously. [...] For knobs and similar "analog" controls, I'd say it takes at least tens of ms before you start to notice any latency. For keys, I personally think it starts to feel weird if the latency approaches 10 ms. More importantly though, latency must be *constant*! A synth that just grabs all pending events once per buffer cycle won't be playable with more than a few ms of latency, as the "random" response times quickly become very noticeable and annoying as the "average" latency increases. If incoming events are properly timestamped and scheduled, this is much less of an issue, and latency has the same effect as varying the distance to the monitor speakers. -- //David Olofson - Developer, Artist, Open Source Advocate .--- Games, examples, libraries, scripting, sound, music, graphics ---. | http://olofson.net http://kobodeluxe.com http://audiality.org | | http://eel.olofson.net http://zeespace.net http://reologica.se | '---------------------------------------------------------------------'