HDSDR and bandwidth

[Adam Nielsen]

Hi all,

I've been experimenting with my P160 for a couple of weeks now but I only just
discovered this group - which is great, as I've been looking for somewhere to
ask my RF newbie questions :-)

Using HDSDR under Windows, I have been able to tune into local FM radio
(WBFM?) but only if I set the HDSDR output sample rate to 96kHz. Any lower
and the sound is very distorted because it's not capturing the full signal.

Why is this? I can understand needing to widen the channel bandwidth to
capture the whole FM signal, but why is this tied in to the sound card's
output sample rate? Aren't the two independent? The channel bandwidth is
used for demodulating, then you could resample that to any given output sample
rate - right?

The reason I ask is that 96kHz isn't quite enough to demodulate the complete
FM signal, but it's the highest setting supported by HDSDR. What I am after
is a way of setting the channel bandwidth to say 120kHz and then resample it
to 48kHz for sound card output.

Is this a limitation of HDSDR or am I misunderstanding how it all works?

Thanks,
Adam.

[Bob R]

Hi Adam,

I believe the output sample rate has to do with the IQ sample rate used for the filter bandwidth rather than the soundcard playback sample rate.  I don't know if that's correct though, I'm just starting myself.

I did discover that WRplus seems to be a bit more well equipped for broadcast FM demodulation.  It's basically a fork (?) of winradhd/hdsdr, and supports ExtIO in much the same way.  Install, copy extio_usrp.dll to the install directory, start, tune near the station, left click on FM then right click on FM.  That will pop up the menu of FM options..pick 192kHz and tinker with stereo/rds/AFT/etc.

WRplus has some usability issues with the UI, especially when the ExtIO dialog box hides behind it, but it does to a great job with FM.

[Miguel A. Vallejo]

Adam Nielsen wrote:

>>> Using HDSDR under Windows, I have been able to tune
>>> into local FM radio (WBFM?) but only if I set the HDSDR
>>> output sample rate to 96kHz.  Any lower and the sound
>>> is very distorted because it's not capturing the full signal.

I tried to listen FM broadcast signals with HDSDR and I was not able to do it even at 96kHz output rate and widening the output spectra to the maximun. The bandwidth is still too small for a common stereo signal.

The only program really suitable for FM signals is WRPlus. I feel HDSDR focused on HF signals, where you will not find nothing really wide, but it's only my opinion. ( But at the other side HDSDR is the only program running in a window. I hate full screen programs like WRPlus or WinRad )

[Adam Nielsen]

Thanks all for the replies! I had an off-list reply suggesting WRPlus as
well. Just to be clear though, I'm not interested in receiving FM broadcasts
with HDSDR, I just want to know why adjusting the playback sample rate affects
how much bandwidth gets demodulated.

> I tried to listen FM broadcast signals with HDSDR and I was not able to do it
> even at 96kHz output rate and widening the output spectra to the maximun. The
> bandwidth is still too small for a common stereo signal.

I am able to listen to local FM stations like this, although obviously mono
and with a bit of noise. But I can clearly hear the music and talking, and
there is no noise if the station is broadcasting at a lower volume.

> The only program really suitable for FM signals is WRPlus. I feel HDSDR
> focused on HF signals, where you will not find nothing really wide, but it's
> only my opinion. ( But at the other side HDSDR is the only program running in
> a window. I hate full screen programs like WRPlus or WinRad )

How does WRPlus capture the whole signal? Bob's message suggests you have to
set something to 192kHz but my sound card won't output 192kHz (and HDSDR
doesn't offer that as an option either.) Does this mean channel bandwidth ==
output sample rate is just a quirk in HDSDR, rather than software radio in
general?

Cheers,
Adam.

[Balint Seeber]

In WRplus, you can right-click on the 'FM' button, and select one of the last two options (mono FM, or stereo - both 192 kHz). In these modes, and 'Medium', the demod bandwidth is not linked to the output sample rate (i.e. it's done 'properly').

In HDSDR, as you say, it seems to be just a quirk. I guess when they add proper WBFM demod, it will become unlinked.

So, in general, you can capture as much as you like/can, and then filter down as far as you wish. Of course, as you mention in your OP, you are still limited by the maximum bandwidth supported by your output device, e.g. soundcard (if you are using one at all).

[LC]

Hi all,

just for clarification: WRplus and HDSDR are both based on Alberto di Bene (I2PHD)'s Winrad version 1.32. WRplus is not a fork of HDSDR.

HDSDR had it's focus on User Interface.

Some guessed right: "Wide" FM is just a quirks in HDSDR: setting output samplerate to 96 kHz is just a workaround to get it to work - somehow.

regards,

LC