Hi Miller,
this sounds great! First-class multi-channel support would be a real
game changer.
Actually, after Winfried Ritsch told me about the "pd_snake" project, I
came up with a couple of ideas on my own. You can find them here:
https://git.iem.at/pd/pdsnake/-/blob/master/docu/discussion.txt. Don't
know if this aligns with what you are envisioning, but it might give you
some inspiration either way :-)
In particular, I would like to point out
https://git.iem.at/pd/pdsnake/-/blob/master/docu/discussion.txt#L33-41.
This would allow us to create patches where the channel count can be
changed dynamically with a single message!
Also, multi-channel signals would give us a chance to vectorize DSP
algorithms that are otherwise hard or impossible to optimize. For
example, with modern AVX instructions you can compute 8 oscillators or
IIR filters for the price of 1. (With proper manual loop unrolling, just
like in the "*_perform8" methods, some compilers are able to vectorize
it automatically.)
(one question about this... I _could_ take a sightly bigger risk and put the
last 3 fields ahead of s_refcount, etc, which I don't think anyone should
be using... this would make things look cleaner).
I think this should be fine.
typedef struct _signal
{
int s_n; /* *TOTAL* number of points in the array */
t_sample *s_vec; /* the array */
t_float s_sr; /* *TOTAL* samples per second */
int s_refcount; /* number of times used */
int s_isborrowed; /* whether we're going to borrow our array */
struct _signal *s_borrowedfrom; /* signal to borrow it from */
struct _signal *s_nextfree; /* next in freelist */
struct _signal *s_nextused; /* next in used list */
int s_vecsize; /* allocated size of array in points */
/* *** NEW STUFF *** */
t_float s_rate; /* sample rate */
int s_length; /* number of points in each channel */
int s_nchans; /* number of channels */
int s_overlap; /* number of times each sample will appear */
}
Personally, I would keep s_n as the number of samples /per channel/. The
total number of samples is simply s_n * s_nchans. Existing externals -
that do not know about s_nchans - would effectively operate on the first
channel and ignore the rest. Newer multi-channel-aware externals, on the
other hand, may use all the channels.
I also think that DSP objects would need a new API method to create
multi-channel /outputs/. The general idea is that the /input /channel
counts are taken from upstream, but the /output /channel counts are
specified by the object and passed downstream. (There might be objects
where input and output channel count differs; any kind of
merger/splitter/mixer objects comes to my mind.)
I think I have some more ideas/notes in one of my notebooks. I can look
them up and see if there's something useful.
Anyway, I am quite excited about this!
Cheers,
Christof
On 01.09.2022 21:58, Miller Puckette via Pd-dev wrote:
Hi Pd dev -
I'm preparing to rework the DSP network to give tilde objects more control over
their inputs and outputs, for instance allowing for multi-channel signals and to
allow objects to decide for themselves whether to promote float inputs to
signals (so that you don't have to say "+~ 0 to get the faster version, and so
that I can make the hip/lop/bp/vcf frequency and Q inputs available as signals
or as floats).
Of course I mean to make this compatible with existin DSP objects, although for
simplicity I'm going to propose one slightly risky move, changing the size of
the t_signal structure -- as iohannes mentioned a few years ago, this seems
very unlikely to break anyone's tilde objects. The new structure would now
look as follows:
typedef struct _signal
{
int s_n; /* *TOTAL* number of points in the array */
t_sample *s_vec; /* the array */
t_float s_sr; /* *TOTAL* samples per second */
int s_refcount; /* number of times used */
int s_isborrowed; /* whether we're going to borrow our array */
struct _signal *s_borrowedfrom; /* signal to borrow it from */
struct _signal *s_nextfree; /* next in freelist */
struct _signal *s_nextused; /* next in used list */
int s_vecsize; /* allocated size of array in points */
/* *** NEW STUFF *** */
t_float s_rate; /* sample rate */
int s_length; /* number of points in each channel */
int s_nchans; /* number of channels */
int s_overlap; /* number of times each sample will appear */
}
(one question about this... I _could_ take a sightly bigger risk and put the
last 3 fields ahead of s_refcount, etc, which I don't think anyone should
be using... this would make things look cleaner).
For example, the FFT object's outputs should really have a sample rate of 1/N
times the input sample rate, a vector length of 1, and a channel count of N. For
compatibility, I'd take the "s_n" field to just be N, although in the future one
could optionally use s_length as N and run as many DFTs as there are channels.
(This would be incompatible with current practice in wierd situations in which
one ran an fft~ objects into another fft~ objects as input - a real bad idea but
perhaps the only way in vanilla to time-reverse a signal block by block, so I
bet someone is depending on being able to do that :)
Meanwhile, before the DSP routine is called, all signal inputs are populated
with vectors by promoting float inputs to signals, all inputs are guaranteed to
have the same s_n field, and all outputs are automatically generated to match
all the inputs. I want that to be the default option but to allow the object to
access non-matching signals, not-filled-in signals (so that it can schedule
scalar versions, as in "+~", and to take care of generating its own output
signals (which may thus have different sizes from the input signals).
I could then design a "trunk~" object that combines or splits one-channel
signals into multichannel ones, and I could extend +~, etc., to know how to add
one-channel signals to multichannel ones. Also, clone~ could (optionally)
unpack multichannel signals to distribute among copies.
it might also be useful to have the option to ask for the output signals,
if auto-generated, never to reuse the same vector as the input; I guess that
can be provided if there's a demand for it.
I'm thinking this is a big enough and dangerous enough change that I should do
it on a separate branch first. I've got some travel coming up but hope to start
coding soonish.
cheers
Miller
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