RadioShack PRO-2055 scanner discriminator modification

I own one of these scanners. It is a bit long in the tooth now but back in its day (early 2000's) it was the top of the line scanner sold at the now mostly defunct RadioShack. So now that its well out of warranty and without much use for it I decided it was time to do the discriminator audio modification.

This modification will take the pre-filtered raw audio and present it via a socket on the back of the radio for later use by sound card or SDR software. Such uses could be for receiving Weather Satellite data around 137.5MHz. Both APT and LRPT are possible due to the bandwidth available. Some 50KHz of audio spectrum (+/- 25KHz) should be usable thereby overcoming issues of doppler shift. 

Its a simple modification requiring only 4 components; 6 inches of RG174 type coax, a 0.002uF (202) capacitor, a 10K resistor (Br/Bl/Or) and a 3.5mm (1/8") mono panel mounted headphone socket.

Remove the radio from its black enclosure sleeve. Remove the top and bottom covers and disconnect the speaker by pulling the connector from the board. On the component side of the board locate TP4. It is located next to a small 8pin surface mount IC. Follow the black coax in the below picture to locate TP4. Solder the inner of the coax to TP4 and the outer/braid to a suitable ground point. I scratched off some of the solder resist to reveal a large copper ground plane and put the braid there. Run the coax through the large speaker hole to the other side of the board.

Take a look on the back of the radio. You will see a few factory made holes including one marked for an M5 screw. As if by magic, the 3.5mm headphone socket will install just nicely into this hole. No drilling required.

With your socket mounted solder the 0.002uF (202) capacitor across the positive and ground pins. Solder one end of the 10K (Br/Bl/Or) resistor to the positive pin. Finally prepare the remaining end of the RG174 coax and connect the inner to the end of the resistor that is now sticking out from the socket. Connect the braid to the ground pin together with the other end of the capacitor.

Check your work for shorts and dry joints. Reassemble the radio making sure to re-attach the speaker connector. Power up the scanner and test that it still works as before.

It [still] Buggers Other Channels - FM Edition

I recently wrote an article about how IBOC (known by its commercial name of "HDRadio") causes interference to other broadcasters. That article was based on the AM/MW band but you'll be interested to hear that it is also the case on the FM band too.

I live in the greater Philadelphia area. A scan of my local FM band reveals some 23 radio stations broadcasting both an FM signal and an IBOC signal.

IBOC in "wrapper" mode

In the screenshot to the left we can see how the digital radio transmission is wrapped around the main analog (FM) carrier.  The IBOC signal (properly known as "In Band On Channel") is seen here in its "wrapper" mode. This is where the data carrier is split into 2 segments and sent out on different frequencies separated by the analog FM signal.

The IBOC system uses a COFDM type transmission method and so the data is in fact on lots of frequencies all at the same time. That there is a gap in the middle of it does not matter. The decoder simply skips over the gap looking for more data carriers. 

The maximum data throughput for this mode is 120kbps. If there were no analogue FM carrier in the middle and the gap was filled with more data the throughput increases dramatically to several Megabits. At that speed we are able to transfer TV images! This is only available on the FM band due the the wider bandwidth of the signals. The AM band is limited to a total of 20kbps.

Similar to our discussion in my previous article, here in the USA radio stations are given a fixed set of 100 KHz wide frequencies spaced out every 300KHz for every radio "market". This allows markets that butt up against each other to coexist interference free. Well, that's the theory at least.

Among my many (too many?) radio receivers is an RTL-SDR. This is a USB stick that is capable of receiving radio signals from about 30MHz up to 2GHz. By computer sampling a received radio signal and then pushing that sample through some software we are able to decode the information it carriers. This is known as Software Defined Radio. 

For this article I connected my RTL-SDR to one of my many (too many?) outdoor antennas and performed a scan of the FM radio band. As noted above I received 23 stations transmitting IBOC data. I used a combination of "GQRX" (Linux) and "RTL-SDR FM-Radio" (Android) as well as the RTL-SDR driver software for the USB device itself. These are all open source applications and are freely available. 

GQRX SDR software showing a few Megahertz of spectrum including some FM/IBOC stations

In the below picture we can see just how offensive IBOC is. We are looking at 3 FM/IBOC stations side by side. As you can see there is no room in between the stations. There should be a gap here. In this gap should be a stations from the adjacent radio markets of NYC, Wilmington, Scranton, and Lancaster.

IBOC shuts out its neighbours.

So, what is this data carrying anyway? Well, obviously its a digital version of the analogue output. But its so much more than that. Many broadcasters operate more than one media stream. Some of them relay their AM broadcasts, others a partner station from somewhere else on the band. Others yet carry special interest media stream such as Russian language or LGBTQ+ tailored music.

WIOQ 102.1 has 3 media streams

Here we see WIOQ on 102.1MHz. You can see that the stations transmit not only the audio stream but often pictures too. Pictures of the artists, album covers. Often head shots of the DJ also.

And that's not all. Radio stations are the traditional source of news, weather and travel information. Every 10 minutes during drive time you can hear them announce the traffic jams on the Schuylkill Expressway. But instead of just hearing about it how would you like to see a map?

Traffic map

Of course, I'm using an application on my computer to display all this information. But most people will be receiving this in their cars, So this map would pop up on the radio's display. Weather maps too. Talk about distracted driving!!

But has this method of media transfer been surpassed by your cellphone? I think so. It is not uncommon for cars today to have a significant display ability in the dashboard that is tied to your phone. For the most part when you you want traffic information you simply turn on your navigation app. You get up-to-the-second traffic and weather data displayed along side your navigation instructions. Has IBOC's time been cut short? Is it a modern dinosaur? Was it ever really useful in the first instance?