Re: how to interpret tell_off, and the right way to use sox_seek

[Jan Stary <hans@stare.cz>]

> I do take your message to imply that SoX will read no more samples
> than the number of in a file total, so that, e.g., if you read the
> same 10 blocks of samples and sox_seek() to the beginning of them
> repeatedly, eventually your reads will fail.  And in fact, if the
> length of the file is 50 blocks, those reads will fail as soon as you
> have done this 5 times.  If this is not correct, please let me know!!
> :)

There is a difference between SoX the utility and libsox the library.
I am not sure if SoX itself ever reads a block of audio more than once
(I can imagina situations where it would), but that's not a constraint
on what the libsox library can do. If it has a sox_seek(), then I suppose
it does what the name says; in particular, it lets you read the same
block of audio over ano over again as long as you keep seekign back.
(Disclaimer: I have never used libsox directly,
I only use SoX the binary.)

> Now, in answer to your three questions and suggestion about libsndfile:
> 
> (1) In fact, i didn't need to read the same samples multiple times, i
> just needed to use them multiple times.
> And it was possible for me to implement my own buffering scheme.
> And that's what i ended up doing,
> because in this particular case there is a pattern of access that
> repeats.  So it was possible to code around not being able to seek
> freely.

Well, that changes the whole premise.
Is there anything left of the original problem then?

> However it would have been easier not to do any buffering, and just
> seek, and use the implicit buffering of the system.

Using the same data again does not imply any buffering by itself.
You sox_read() into an array and then use it as you wish
(such as play it seven times).

> A sort of analogous situation is with the file system and linux: if
> you're reading through a file the OS will keep it in memory so you
> don't have too much overhead just relying on the OS itself to seek
> back and forth in a file --- the caching has already been done.

You already _have_ the data in memory once you have sox_read() them,
so I fail to see the analogy. Why would you seek back and read it
again _from_the_file_ once you have it in an array?

> (2) I'm doing this programmatically because i anticipate running the
> program many times, and i want to be as sure of correctness as i can.
> (In fact, i'm not actually programming it in C, but i'm using the C
> calling conventions, and in principle i could certainly do this in C.)

What makes you think that writing your own code will make
your use of SoX more "correct"?

> (3) I'm using SoX to do this because it is so good :) :).  I also have
> experience programming in SoX, although obviously i'm still learning.

I still have no idea about what you want to do, eventually.
You are asking about technicalities; what are you actually _doing_ ?

> (4) libsndfile would definitely have been a possibility, but every
> library has trade offs.  As i understand it, libsndfile does not yet
> support mp3, although that is on the road map.  And it is possible
> that there would be some other move that i was making that wouldn't
> work in libsndfile.  So i'd have to recode to see --- although as far
> as i can tell, libsndfile and SoX are both excellent pieces of
> software.

I still have no idea about what you want to do.

	Jan


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