Re: [v10 0/4] media: vidtv: Implement a virtual DVB driver

From: Daniel W. S. Almeida
Date: Fri Sep 11 2020 - 08:26:00 EST

Hey Mauro,

> Thanks for all the hard work on it. Very much appreciated!
> I finally found some time to test it. For now, just a quick
> test from my side, without passing any arguments to the
> driver.

That's nice!

> My plan is to write some patches on the top of yours, in order to
> address the problems I'll find on it. If not something more critical
> won't be solved in time, we may still add it at staging/media.
> Let's see.


> 3. dvbv5-zap wrote an empty audio file (without -P flag).
> Probably there are still some issues at the program
> channel descriptor or service;

I don't remember whether I tried this. I tried dumping the stream to a
file with dvbzap, which should work. By the way, I guess we should be
comparing the output to this

$ ffmpeg -f lavfi -i sine=frequency=1000:duration=5 -ac 2 -c:a s302m
-strict -2 out.ts

Since it produces a playable transport stream file that actually sounds
like a sine tone.

Inspecting ffmpeg & vidtv output side by side in dvbinspector, you'll
see that they're mostly the same. I have a separate PID for the PCR and
some other minor differences.

> 4. The provider service field is null. Perhaps we could
> add some string there, like "".
> 5. Maybe we could also add a simple NIT table, just to
> avoid dvbv5-scan to wait for it until timeout.
> Also, it probably makes sense to add a debugfs interface in
> order to allow injecting errors at the stream at runtime.

Sure. This is fun, sign me up for it.

As I said in a previous email, I think the buffer in vidtv_s302m.c is
not exactly what we want. It sounds like noise.

I got it from here:

I will just copy what I had in a previous email here:

>How do I compute the values for a sine wave such that it can be played indefinitely?
>I was thinking about this: (just spitballing here..)
>sampling_rate_khz = 48000
>buffer_size = sampling_rate_khz * seconds
>i = 0
>for i < buffer_size:
>buffer[i] = 32767 * sin( 2 * PI * freq / (sampling_rate_khz * i) )
>but rather I want to precompute a single period for a given frequency
>such that I can loop on the array indefinitely

By the way, after some time trying out stuff, I guess this is actually
what we need in vidtv_s302m_write_frame:

static u32 vidtv_s302m_write_frame(struct vidtv_encoder *e,
u16 sample)
u32 nbytes = 0;
struct vidtv_s302m_frame_16 f = {};
struct vidtv_s302m_ctx *ctx = e->ctx;

/* from ffmpeg: see s302enc.c */

u8 vucf = ctx->frame_index == 0 ? 1 : 0;[0] = reverse[sample & 0xff];[1] = reverse[(sample & 0xff00) >> 8];[2] = (vucf << 4) | (reverse[(sample & 0x0f)] >> 4);[3] = reverse[(sample & 0x0ff0) >> 4];[4] = reverse[(sample & 0xf000) >> 12] >> 4;

nbytes += vidtv_memcpy(e->encoder_buf,

e->encoder_buf_offset += nbytes;

if (ctx->frame_index >= S302M_BLOCK_SZ)
ctx->frame_index = 0;

return nbytes;

i.e. see:
Figure 5, page 4.

Lastly, I am somewhat new to C, so a few things might look odd here and
there. If you come across something that's too verbose tell me and I
will try to tidy it up.

-- Daniel