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).
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 :)
Christof
On 04.03.2024 06:23, Christof Ressi wrote:
Hi,
making it seem it would load the file into an internal memory and not
just read directly from the disk
The help file says "soundfile playback from disk"...
Here's how the object works:
A worker thread is reading data from the given file and fills a
buffer. If the buffer is full, it waits until there is space. The
thread starts to do its job right after the [open( message.
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.
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.
(BTW, I have no idea why the help patch uses a buffer size of 1 MB...)
---
[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.
Christof
On 04.03.2024 05:24, Alexandre Torres Porres wrote:
Hi, a discussion on facebook led me to revise the help file of
[readsf~] and I'm making some edits and changes. It seems it wasn't
all to clear how it works, making it seem it would load the file into
an internal memory and not just read directly from the disk, and in
fact I'm not really sure how it works.
Also, the help has been saying forever how one should open a file "a
bit" in advance, which is vague and it also doesn't make it clear why
and how long soon... it also says it starts reading from the file
right away, but doesn't play it until you say so... this is what
makes it a bit confusing to people I guess, cause it seems to load
into memory. Now, I believe there is some operation that is done in
advance and then it adds some latency maybe, so if you do it in
advance you get to manage this a bit better, is that it?
Can we have and add a bit more information about it?
Also, is it the case that this used to be a somewhat significant
issue back in the day, which means this has become not significant
all for a while?
Last, but not least, I can't make much sense of the 2nd argument. I
had a look at the code to see what is the default value (which is
also the minimum allowed value), something I always put on the help
files, and I wonder if this is some kind of memory buffer that we
load the file into... Moreover, why would someone need a bigger
buffer than the default value?
I need this information well sorted to make this help file as nice as
the others.
Cheers!
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