On 07.03.2024 01:27, Alexandre Torres Porres wrote:
Em seg., 4 de mar. de 2024 às 02:25, Christof Ressi info@christofressi.com escreveu:
A worker thread is reading data from the given file and fills a buffer.and this buffer size is set via the 2nd argument, right?
Yes!
If the buffer is full, it waits until there is space. The thread starts to do its job right after the [open( message.And it'll have space when it starts playing, which frees the data from the buffer, huh?
Yes, although "frees the data" is the wrong term. The perform routine reads a range of samples and advances the read pointer, after which these samples can be *overwritten* by the worker thread. It's just a simple ringbuffer.
Once we send the [start( message, the perform method simply tries to read a block of samples from the buffer and copy it to the outlets. If there is not enough data in the buffer, the method blocks - which is something we definitely want to avoid! This is exactly the reason why we need to wait a little bit between the [open( message and the [start( message; otherwise the perform routine might have to wait for the buffer, causing a dropout.But it's not like we need for the whole buffer to get filled before playing, right? So we can read and free from the buffer while it's being filled.
Yes! Although in practice the buffer will be filled pretty quickly once the soundfile has been opened. One could say that the perform routine "drives" the worker thread.
The second argument for [readsf~] is the size of the buffer. The default value seems to be 262144 bytes (per channel). In single-precision Pd that corresponds to 65536 samples, which should be more than enough. I think this value comes from the times where everybody had slow HDDs with unpredictable seek times; for modern SSDs it can be much smaller, but we probably don't care about a few kilobytes.I see.
(BTW, I have no idea why the help patch uses a buffer size of 1 MB...)seems weird in fact to require anything bigger than this, really, and this should be clear in the help file! When should one care and feel like 65536 isn't enough?
Pretty much never.
[writesf~] behaves just the otherway round. Since the perform routine is the producer, we don't need to wait after the [open( message. But after we send [stop(, we should to wait for the worker thread to drain the buffer and write the data to disk before we send another [open( message.maybe worth mentioning.
Actually, I forgot something important: Of course, the worker thread must also *open* the file! If the file is not yet cached by the OS, this can indeed take a few milliseconds.If you don't add some delay between "open" and "start", you might notice that you get a dropout the very first time, but not on subsequent times. In fact, if you don't wait between "open" and "start", the perform method almost certainly blocks. However, often we don't notice because it may be "absorbed" by Pd's own ringbuffer (= "Delay" in the audio settings).Thing is, I don't think I've ever noticed a dropout and have been ignoring this warning and playing files right away forever, since I started using Pd in the 2000s... I got this rather old 2013 macbook air, and using it with a delay of only 5ms doesn't give me any noticeable dropout!
I did and still do notice it! Opening a file (that is not cached by the OS) can take a few milliseconds. If your delay setting is low and CPU load is high, you can easily get a dropout.
Can others test this?
This is why I was assuming that it was a concern for pretty old computers. I thought maybe it was a cpu issue, but I see it could also be hard drive related now. This ver old macbook air has got SSDs anyway...
Anyway, I agree that the help needs some more clarification! (Just make sure you really understand how the object works before changing the help patch :)that's what I am doing :)
+1