Re: Reflections on writing a sequencer

Hi  -
     Of course, you would want to generate events as short a time in to 
the future as possible in order to stay responsive to rate (or tempo) 
changes.
     Ideally a JavaScriptAudioNode could be used as the event generator. 
It's onaudioprocess() method could check the length of the output buffer 
it is passed, and do nothing else but call "note on" events for other 
nodes it wants to play within that short period of time.
     I haven't tried that yet, but would noteon events be handled 
properly when generated in this "just in time" manner? Would this be a 
violation of protocol to use a onaudioprocess() as what would amount to 
a rock-solid sample-accurate periodic callback function?

Best,
              - lonce



On 25/7/2012 12:40 AM, Joseph Berkovitz wrote:
> HI Adam,
>
> I think one general way to structure sequencer playback is as follows 
> -- I've used this approach with WebAudio successfully in the past:
>
> 1. Just before starting playback, take note of the AudioContext's 
> currentTime property.  Add a small time offset to it, say 100 ms.  The 
> result will be your performance start time, corresponding to time 
> offset zero in your sequencer data.  (The time offset provides a short 
> window in which to schedule the first events in the sequence).
>
> 2. Create a scheduler function that will run periodically, which 
> examines the AudioContext's currentTime and subtracts the previously 
> captured startTime. That gives you a "current performance time" at the 
> moment the callback occurs, expressed in terms of your sequencer 
> data.  Then create AudioNodes representing all sequencer events that 
> occur within an arbitrary time window after this current performance 
> time (say, several seconds) and schedule them with noteOn/noteOff.
>
> 3. Call the function immediately, and also use setInterval() or 
> setTimeout() to schedule callbacks to the above function on some 
> reasonable basis, say every 100-200 ms. The exact interval is not 
> important and can be tuned for best performance.
>
> This approach is relatively insensitive to callback timing and in 
> general allows audio to be scheduled an arbitrary interval in advance 
> of its being played.
>
> ...Joe
>
>
> On Jul 24, 2012, at 11:40 AM, Adam Goode wrote:
>
>> Hi,
>>
>> Yesterday I tried to write an extremely simple sequencer using 
>> webaudio. My goal was to have a tone play periodically, at a 
>> user-selectable low frequency interval.
>>
>> The main problem I ran into was the difficulties in scheduling events 
>> synchronized with the a-rate clock.
>>
>> If I want to play a tone twice per second, I want to call this code 
>> in a loop, indefinitely:
>>
>> var startTime = ....
>> var o = c.createOscillator();
>> o.connect(c.destination);
>> o.noteOn(startTime);
>> o.noteOff(startTime + 0.1);
>>
>> I can't just put it in a loop, I need to schedule this in a callback, 
>> when necessary to fill the event queue. But what callback to use? 
>> setInterval is not appropriate, since the setInterval clock will skew 
>> quickly from c.currentTime. And busy looping with setInterval(0) will 
>> consume a lot of CPU and gets throttled when switching tabs (try 
>> putting the drum machine demo in a background tab and see).
>>
>> My solution was this:
>>
>> var controlOscillator = c.createOscillator();
>> controlOscillator.frequency.value = 2;
>> var js = c.createJavaScriptNode(256, 1, 0);
>> controlOscillator.connect(js);
>>
>> js.onaudioprocess = function(e) {
>>   ... detect positive zero crossing from control oscillator ...
>>   if (zeroCross) {
>>     var o = c.createOscillator();
>>     o.connect(c.destination);
>>     var startTime = ... zero crossing offset + playbackTime ...
>>     o.noteOn(startTime);
>>     o.noteOff(startTime + 0.1);
>>   }
>> };
>>
>>
>> This does work (except for missing playbackTime 
>> https://bugs.webkit.org/show_bug.cgi?id=61524, needing to connect the 
>> javascript node to destination, and another bug on chrome 
>> http://crbug.com/138646), but is awkward. There is also the question 
>> of having a disconnected graph: I am sending control data, not audio 
>> data, so I don't want to connect it to destination.
>>
>> I essentially want to have a callback for getting new control data, 
>> to keep the event pipeline filled without overflowing any noteOn 
>> buffer or falling behind. Is the javascript node appropriate for 
>> this? I feel like there could be something more explicit, like a 
>> setInterval off of the audio context.
>>
>>
>>
>> Adam
>>
>>
>
> ... .  .    .       Joe
>
> *Joe Berkovitz*
> President
>
> *Noteflight LLC *
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