Mailing List Archive

On Mon, 2009-11-30 at 23:41 -0800, Jeff Campbell wrote:
> Hello All,
>
> I was using the ATSC tuner side of the HVR-1600 over the weekend and I
> had a hard time determining my signal strength or finding a place
> where I can get a consistent SNR reading that can be used to compare
> the relative strength of the various channels I am able to tune. What
> is the recommended way to determine signal strength and SNR?

I'm sure Devin has a lot more details than I, but you can look at the
output of femon. Keep the following in mind:

1. The status is coming from the s5h1409 module, the driver module for
the ATSC/QAM demodulator

2. The signal strength is bogus. From the code it looks like a copy of
the SNR.

3. The SNR value will only be valid when there is a lock. Don't count
on it otherwise.

4. Since Steve Toth wrote the driver, I'm assuming the SNR value is in
0.1 dB, but I'm not sure.


That of coure still doesn't give you what your margins are for good,
bad, and marginal for 8-VSB, QAM-64 and QAM-256. I suppose one can look
up the theoretical BER vs SNR curves to find out.

That's actually one status output I would like: SNR margin. The
absolute values don't mean much to people.

Regards,
Andy

> -Jeff



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On Tue, Dec 1, 2009 at 6:33 AM, Andy Walls <awalls@radix.net> wrote:
> On Mon, 2009-11-30 at 23:41 -0800, Jeff Campbell wrote:
>> Hello All,
>>
>> I was using the ATSC tuner side of the HVR-1600 over the weekend and I
>> had a hard time determining my signal strength or finding a place
>> where I can get a consistent SNR reading that can be used to compare
>> the relative strength of the various channels I am able to tune.  What
>> is the recommended way to determine signal strength and SNR?
>
> I'm sure Devin has a lot more details than I, but you can look at the
> output of femon.  Keep the following in mind:
>
> 1. The status is coming from the s5h1409 module, the driver module for
> the ATSC/QAM demodulator
>
> 2.  The signal strength is bogus.  From the code it looks like a copy of
> the SNR.
>
> 3. The SNR value will only be valid when there is a lock.  Don't count
> on it otherwise.
>
> 4.  Since Steve Toth wrote the driver, I'm assuming the SNR value is in
> 0.1 dB, but I'm not sure.
>
>
> That of coure still doesn't give you what your margins are for good,
> bad, and marginal for 8-VSB, QAM-64 and QAM-256.  I suppose one can look
> up the theoretical BER vs SNR curves to find out.
>
> That's actually one status output I would like: SNR margin.  The
> absolute values don't mean much to people.

Everything Andy said above is correct. If you have any further
questions that Andy's response didn't answer, feel free to ask.

Devin

--
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com

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Thanks for confirming things Andy, Devin.

I am getting 21.5 to 22 on SNR, which seems quite low. This is for OTA
signals. I'm used to 25++ on a cable network. I am quite a distance from
the transmitting tower, and at a low elevation so perhaps it is to be
expected. I'd be curious to hear what others are getting and their relative
distance from the towers if they know it. I'm about 35 miles away.

It seems to be consistent across all three stations, which are all coming
off the same tower as far as I am aware.

Generally the video performance is very good but I get occasional periods of
lost packets and slight pixelation in small areas, which is consistent with
being near the lower end of signal strength and losing a few packets.

At the moment I am using dvbstream to stream the output but I'm finding it
dies with a buffer overflow error after a random period of time. I suspect
this may be related to the signal strength and mangled packets, but I
haven't looked in to it in any detail yet. Is dvbstream normally stable for
long term streaming?

-Jeff

On Tue, Dec 1, 2009 at 3:33 AM, Andy Walls <awalls@radix.net> wrote:

> On Mon, 2009-11-30 at 23:41 -0800, Jeff Campbell wrote:
> > Hello All,
> >
> > I was using the ATSC tuner side of the HVR-1600 over the weekend and I
> > had a hard time determining my signal strength or finding a place
> > where I can get a consistent SNR reading that can be used to compare
> > the relative strength of the various channels I am able to tune. What
> > is the recommended way to determine signal strength and SNR?
>
> I'm sure Devin has a lot more details than I, but you can look at the
> output of femon. Keep the following in mind:
>
> 1. The status is coming from the s5h1409 module, the driver module for
> the ATSC/QAM demodulator
>
> 2. The signal strength is bogus. From the code it looks like a copy of
> the SNR.
>
> 3. The SNR value will only be valid when there is a lock. Don't count
> on it otherwise.
>
> 4. Since Steve Toth wrote the driver, I'm assuming the SNR value is in
> 0.1 dB, but I'm not sure.
>
>
> That of coure still doesn't give you what your margins are for good,
> bad, and marginal for 8-VSB, QAM-64 and QAM-256. I suppose one can look
> up the theoretical BER vs SNR curves to find out.
>
> That's actually one status output I would like: SNR margin. The
> absolute values don't mean much to people.
>
> Regards,
> Andy
>
> > -Jeff
>
>
>

On Tue, Dec 1, 2009 at 12:41 PM, Jeff Campbell <jac1dlists@gmail.com> wrote:
> Thanks for confirming things Andy, Devin.
>
> I am getting 21.5 to 22 on SNR, which seems quite low.  This is for OTA
> signals.  I'm used to 25++ on a cable network.  I am quite a distance from
> the transmitting tower, and at a low elevation so perhaps it is to be
> expected.  I'd be curious to hear what others are getting and their relative
> distance from the towers if they know it.  I'm about 35 miles away.

Bear in mind that the expected SNR for ATSC is different than for
ClearQAM. For ATSC, you generally want an SNR no lower than 25. For
ClearQAM you generally want an SNR no lower than 30. These numbers
should not be any sort of "official standard" - just my general
experience. For the OTA case, I am a pretty crappy use case though,
since I am literally in the shadow of the Empire State building.

> It seems to be consistent across all three stations, which are all coming
> off the same tower as far as I am aware.

Should be easy enough to check - just look at your channels.conf file
and see if they are all coming from the same frequency.

> Generally the video performance is very good but I get occasional periods of
> lost packets and slight pixelation in small areas, which is consistent with
> being near the lower end of signal strength and losing a few packets.

Which application are you using? I have noticed that the different
MPEG2 implementations vary wildly in terms of how well they respond to
even the slightest of errors.

> At the moment I am using dvbstream to stream the output but I'm finding it
> dies with a buffer overflow error after a random period of time.  I suspect
> this may be related to the signal strength and mangled packets, but I
> haven't looked in to it in any detail yet.  Is dvbstream normally stable for
> long term streaming?

I don't have much experience with dvbstream. Typically I use
something like mplayer or Kaffeine to watch the stream, and then run
femon in a separate window.

Devin

--
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com

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On Tue, Dec 1, 2009 at 12:41 PM, Jeff Campbell <jac1dlists@gmail.com> wrote:
> Thanks for confirming things Andy, Devin.
>
> I am getting 21.5 to 22 on SNR, which seems quite low.  This is for OTA
> signals.  I'm used to 25++ on a cable network.  I am quite a distance from
> the transmitting tower, and at a low elevation so perhaps it is to be
> expected.  I'd be curious to hear what others are getting and their relative
> distance from the towers if they know it.  I'm about 35 miles away.
>
> It seems to be consistent across all three stations, which are all coming
> off the same tower as far as I am aware.
>
> Generally the video performance is very good but I get occasional periods of
> lost packets and slight pixelation in small areas, which is consistent with
> being near the lower end of signal strength and losing a few packets.
>
> At the moment I am using dvbstream to stream the output but I'm finding it
> dies with a buffer overflow error after a random period of time.  I suspect
> this may be related to the signal strength and mangled packets, but I
> haven't looked in to it in any detail yet.  Is dvbstream normally stable for
> long term streaming?

One more point worth making - if you haven't already, you should
definitely consider installing the latest v4l-dvb code. There were
fixes made recently which considerably improve the quality of
reception for ClearQAM (you will see a 3-4 dB improvement in SNR).

Devin

--
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com

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On Tue, Dec 1, 2009 at 10:25 AM, Devin Heitmueller <
dheitmueller@kernellabs.com> wrote:

>
> One more point worth making - if you haven't already, you should
> definitely consider installing the latest v4l-dvb code. There were
> fixes made recently which considerably improve the quality of
> reception for ClearQAM (you will see a 3-4 dB improvement in SNR).
>
>
Devin,

Thanks for all the additional info. I'll dig in to it this afternoon. I'm
not surprised my signal is a bit low as I haven't tried an outdoor antenna
yet, that will probably easily move me up over 25 for SNR.

I am only tuning ATSC OTAs at the moment, I don't have cable here, so no
clear QAM for me. Will updating the code base provide any change to ATSC
tuning sensitivity at all or was ATSC tuning not affected by a lower
sensitivity in the driver?

Thanks,

-Jeff

On Tue, Dec 1, 2009 at 2:09 PM, Jeff Campbell <jac1dlists@gmail.com> wrote:
> Devin,
>
> Thanks for all the additional info.  I'll dig in to it this afternoon.  I'm
> not surprised my signal is a bit low as I haven't tried an outdoor antenna
> yet, that will probably easily move me up over 25 for SNR.
>
> I am only tuning ATSC OTAs at the moment, I don't have cable here, so no
> clear QAM for me.  Will updating the code base provide any change to ATSC
> tuning sensitivity at all or was ATSC tuning not affected by a lower
> sensitivity in the driver?

All of the fixes were for ClearQAM tuning. If you *do* see any
difference in ATSC quality, that would be a bug (since all the
optimizations were conditional on doing ClearQAM tuning).

Out of curiosity, which application are you testing with?

Devin

--
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http://www.kernellabs.com

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On Tue, 2009-12-01 at 09:41 -0800, Jeff Campbell wrote:
> Thanks for confirming things Andy, Devin.
>
> I am getting 21.5 to 22 on SNR, which seems quite low. This is for
> OTA signals. I'm used to 25++ on a cable network. I am quite a
> distance from the transmitting tower, and at a low elevation so
> perhaps it is to be expected. I'd be curious to hear what others are
> getting and their relative distance from the towers if they know it.
> I'm about 35 miles away.

I'm getting 26.5 (0x109) dB for 8-VSB OTA. That's with a highly
directional ariel (?? dB), installed in my attic (-10 dB for the
celtex), a Winegard low noise ( < 3 dB NF) high gain ( +29 dB IIRC)
preamp, ~75 miles from the city, cold night, high pressure, clear skies,
full moon so it's too bright to tell if the galactic disc is in the
antenna's beam, channel at 177 MHz, ....

:)

Works OK with occasional uncorrectable error bursts.

> It seems to be consistent across all three stations, which are all
> coming off the same tower as far as I am aware.
>
> Generally the video performance is very good but I get occasional
> periods of lost packets and slight pixelation in small areas, which is
> consistent with being near the lower end of signal strength and losing
> a few packets.

Watch for the unc and ber counts in femon when you get pixelation.
Hopefully they correlate.


> At the moment I am using dvbstream to stream the output but I'm
> finding it dies with a buffer overflow error after a random period of
> time. I suspect this may be related to the signal strength and
> mangled packets, but I haven't looked in to it in any detail yet. Is
> dvbstream normally stable for long term streaming?

No idea.

Regards,
Andy

> -Jeff



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On Tue, Dec 1, 2009 at 11:15 AM, Devin Heitmueller <
dheitmueller@kernellabs.com> wrote:

> All of the fixes were for ClearQAM tuning. If you *do* see any
> difference in ATSC quality, that would be a bug (since all the
> optimizations were conditional on doing ClearQAM tuning).
>
> Out of curiosity, which application are you testing with?
>
>
Hi Devin,

I'm trying a number of different things as I get familiar with the process.
Ultimately we'll be using an internal application that processes and routes
mpeg packets. At the moment though I just want to find a stable and
reliable way to read the stream from the cli. dvbstream likes to buffer
overflow on me, and I haven't checked in to why yet. I've gotten streams 5
minutes to 5 hours in length, but it always exits with a buffer overflow. I
suspect we'll end up accessing the device directly, I just haven't gotten
that far yet.

It works fine in MythTV.

-Jeff

On Fri, Dec 4, 2009 at 3:09 PM, Jeff Campbell <jac1dlists@gmail.com> wrote:
> I'm trying a number of different things as I get familiar with the process.
> Ultimately we'll be using an internal application that processes and routes
> mpeg packets.  At the moment though I just want to find a stable and
> reliable way to read the stream from the cli.  dvbstream likes to buffer
> overflow on me, and I haven't checked in to why yet.  I've gotten streams 5
> minutes to 5 hours in length, but it always exits with a buffer overflow.  I
> suspect we'll end up accessing the device directly, I just haven't gotten
> that far yet.

The -EOVERFLOW return for reading a DVB stream means that the
application did not read the DVB device fast/often enough, and as a
result the DVB core dropped some packets. It's safe to continue
reading, but bear in mind that your stream will have an interruption
in it. You might need to increase the priority of the dvbstream
process to ensure that it doesn't get interrupted by other things
going on in the machine.

Devin

--
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http://www.kernellabs.com

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