[App_rpt-users] app_rpt from MP3 stream was remote receivers questions

David McGough kb4fxc at inttek.net
Sat Mar 5 15:24:55 EST 2016

Hi Guys,

The SDR solution for a multi-receiver site is interesting. Here are some 
observations I've made, experimenting with RTL-SDR and AirSPY hardware.

First, as mentioned earlier, the very cheap ($9 from China) RTL-SDR has no
front-end filtering, very minimal RF Dynamic Range (perhaps 50db usable?
--an 8-bit A/D), a very noisy PLL and poor weak-signal performance...They
probably won't fly too well when in the presence of multiple signals which
vary significantly in strength. When these SDR's hear a loud signal, their
RF gain must be turned down (many do this automatically in software) to
keep from getting "crunched." When that happens, the weaker signals
disappear in the noise!  ....But, hey, for $9, they're pretty neat -TOYS-
!! And, they're a great way to start learning about the various SDR
software available, much of this software also supporting higher-end 

Next, there is a generation of greatly improved wide-band SDR's out there, 
such as the AirSPY, which perform MUCH better (about 80db dynamic range, 
12-bit A/D). There are several competitors in this performance category 
and it's evolving rapidly. IMO, the AirSPY is quite good; I've enjoyed it. 
(No, I don't have any affiliation with that Company, except that I've got 
a version R2 radio from them). As for a true remote RX, I don't know. But, 
I'm getting prepared to try it! However, be aware that processing a large 
segment of spectrum takes substantial CPU horsepower--more than my laptop 
can provide! So, I end up with audio chops unless I reduce the amount of 
spectrum I'm monitoring....Also, since the dual-core CPU is staying busy 
running DSP software, it's using significant power (and is HOT).

Now comes a complex issue to handle--variable delay in the RF-to-audio 
conversion. If you're running a standard analog RX (like the GM300 
mentioned), the delay through the radio is minimal and CONSISTENT between 
radios--ideal for voting receivers, etc. The SDR delay is large, perhaps 
as much as 0.5 seconds, depending on the various DSP processing and CPU 
speed. Also, as implied, the SDR decode speed is variable, depending on the 
DSP options used, number of streams decoded, etc. This makes syncing the 
audio for voting very difficult (or impossible?).

Anyhow, I think you'll have a lot of headaches trying to making the 
current crop of SDR's perform as voting receivers, particularly if you 
mix analog receivers and SDR's....Now, as a SINGLE remote-base RX, the SDR 
concept is very interesting. But, for voting, I'd go with the GM300's (or 
similar), and not even consider an SDR at this point in their evolution.

Just my $0.02.

73, David KB4FXC

On Sun, 6 Mar 2016, Steve Wright wrote:

> On 05/03/16 06:00, mike at midnighteng.com wrote:
> > RTL-SDR have no front-end rejection. While I like to use them for many things, They
> > make terrible choice in repeater work.
> No receiver has any front end rejection until you had a front end, which 
> we'll do.
> > If your RPTR TX'r is in the same band, watch it totally de-sense that RTL-SDR.
> No, as stated, these are remote receivers.  There's no other noise 
> generating equipment on site except the solar panel regulator.
> > What good would it be.
> > Better off with a tyt/boafeng HT etc. And that's not giving those any credit.
> Not a silly idea actually.  A simple coax front end cavity,  or notch to 
> target the image might be in order.
> > You do need some scheme to vote/select the most valid one (to the core site) if it
> > is different PL etc.
> All built in to app_rpt isn't it?
> >
> > FYI There are RTCM /voter systems for app_rpt if you want to do it right.
> Too expensive when building a three repeater, six receiver distributed 
> system.  That's 24 RTCMs.
> > Since there is a left and right definable audio channel, I had always thought you
> > could double-up on the dongles but never tried it.
> My understanding is that cannot be done.  Dongles are cheap as chips, so 
> that's false economy.
> My original query as follows ;
> > For remote receivers, is there any work being done on using RTL SDR
> > rtl_fm multiple-instances on one dongle, as multiple app_rpt IP
> > connected receivers?
> >
> > I ask, because it seems that a remote (RX only) site can host as many
> > receivers as I can shoe-horn in the box, and as many as I can add
> > solar-panels to feed.
> >
> > It also seems, that I may as well host neighboring areas' remote
> > receivers, and they host mine - for even larger coverage.
> >
> > It just seems a stack of ten GM300s in there is the hard way, and it's a
> > lot of current draw too.
> >
> There is a group of aviation-monitoring enthusiasts who have cracked the 
> multi-channel thing, so it seems to be just a matter of feeding multiple 
> MP3 streams to multiple app_rpt daemons.
> Anyone know how this works?
> S
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