[Linux-kernel-mentees] [v10 0/4] media: vidtv: Implement a virtual DVB driver

Daniel W. S. Almeida dwlsalmeida at gmail.com
Fri Sep 11 12:18:20 UTC 2020

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 "linuxtv.org".
> 	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;

	f.data[0] = reverse[sample & 0xff];
	f.data[1] = reverse[(sample & 0xff00) >>  8];
	f.data[2] = (vucf << 4)  | (reverse[(sample & 0x0f)] >> 4);
	f.data[3] = reverse[(sample & 0x0ff0) >>  4];
	f.data[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: https://docplayer.net/37828038-For-television-mapping-of-aes3-data-into-an-mpeg-2-transport-stream.html
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

More information about the Linux-kernel-mentees mailing list