15
\$\begingroup\$

What is a cheap way to receive and record on PC several (tens) of fm-radios (public, e.g. news, music, etc)? Such radios uses frequencies like 90.4 MHz or 102.7 MHz, sends a stereo (sometimes with digital text subchannel).

If I want to record tens of radios, I need to cover a spectrum of tens of MHz.

The first way is to buy two or three dozen usual radiorecievers and try to connect them all to PC (e.g. with half-dozen of multichannel external usb soundcards). I think, that this is not very cheap.

The second way is to build a single radio, which will downsample entire band 88-105 MHz into 3-20 MHz, then feed it into high-speed ADC, and do a software detection of each station.

Is the second way possible? cheap? How mush will it cost?

What hardware can be used, if I have no skills of making PCBs and using ADCs & FPGAs?

\$\endgroup\$
  • 1
    \$\begingroup\$ The first way might be doable. As of early 2013, there are $30 DVB-T USB receivers that, while low end, will receive loud signals like FM broadcast just fine. A single receiver can record a 2 Mhz slice of VHF spectrum. So in principle, you need 10 of these: $300 for the receivers. Getting one or more computers to organize and record all of the files using gnuRadio is left as an exercise to the reader. \$\endgroup\$ – Paul Mar 6 '13 at 4:19
  • \$\begingroup\$ Check the HackRF =USB2.0 20MSPS, 8bit rtl-sdr.com/hackrf-sdr-giveaway and bladeRF =USB3.0 40MSPS, 12 bit (300MHz to 3.8GHz oob,converter needed for 90MHz) \$\endgroup\$ – osgx Aug 6 '13 at 18:11
14
\$\begingroup\$

The second technique you mention is the way to do it, using what is called a "Software Defined Radio" or SDR. Many radio amateurs are using SDRs, and the simple ones are very cheap, about 30 dollars for a kit that down-converts the input into in-phase and quadrature baseband audio output which is fed into the stereo inputs of a PC sound card for digital signal processing. However, they are using relatively low-frequency signals on the HF amateur radio bands, and the hardware doesn't use any exotic components. Digitising VHF signals as you require and receiving several channels simultaneously is going to be rather expensive, the ADC alone is going to cost about 50 dollars and you will also need an FPGA and a DSP, unless you convert down to baseband and do the DSP on a PC. You will need a lot of high-frequency design experience, be able to develop code for the FPGA, write DSP code and be able to design a high-speed multilayer PCB, so you should start studying. :)

As for cost, I'd estimate 500 dollars for the hardware, including the PCB, assuming you designed it yourself.

Linear Technology makes suitable ADCs that can downsample at 750 MHz! They were good enough to give me a couple as free samples. I have suitable FPGA and DSP boards, so it's just a question of putting them together. :)

\$\endgroup\$
  • 3
    \$\begingroup\$ @Leon, i disagree with a bit of this. I have a few friends whom do this regularly. The cost for doing this project is not primarily influenced by what frequency he is at, but what frequency bandwidth he wants for his signals. I can build a VCO and mixer that goes to 2.4GHz for only 10 or 20 dollars more then one that stops at 10MHz. The ADC to handle 20MHz of sample space, now that is a problem. \$\endgroup\$ – Kortuk May 9 '11 at 8:07
  • \$\begingroup\$ That's why the professionals use a very fast ADC at the front end and don't bother with a mixer! \$\endgroup\$ – Leon Heller May 9 '11 at 8:22
  • 4
    \$\begingroup\$ @LeonHeller, The ADC is what costs so much, and a 10GHz adc is not really reasonable except at very very high cost. I have not heard of SDRs not using a mixer. The more expensive ones my friends have also still use a mixer. \$\endgroup\$ – Kortuk May 9 '11 at 8:36
  • \$\begingroup\$ Many of the commercial ones, up to 500 MHz or so, just use an ADC. A mixer will be needed at higher frequencies, of course. It just isn't needed below 500 MHz. \$\endgroup\$ – Leon Heller May 9 '11 at 9:25
  • 1
    \$\begingroup\$ oscilloscope is not for long continous recording, they usually can grab only several milliseconds (if grabbing at high rate) \$\endgroup\$ – osgx May 9 '11 at 11:50
5
\$\begingroup\$

Actually, RTL-SDR can solve this in ... several 20 USD dongles, each covering up to 3 MHz of FM spectrum (I achieved clear reception of 4 or 5 FM stations with 0.4 MHz spacing per dongle).

As was found by osmocomSDR project, there are lot of Chinese DVB-T USB receivers, built on Realtek RTL2832U chip and some 50MHz - 1+ GHz I/Q radio frontend. This chip has hardware detector and decoder for DVB-T video signals and puts raw MPEG TS to the PC. But there is a feature when chip transfers all I/Q samples to the host. Using USB 2.0 it can transfer to 3.2 or 2.4 MS/s (millions of I/Q samples per second) and there are now several programs to do SDR processing of samples. There is even GNURadio integration.

\$\endgroup\$
4
\$\begingroup\$

For your first solution you shouldnt need a sound card. They make usb fm receivers. Like this:
http://www.amazon.com/ADS-RDX-155-EF-Instant-FM-Music/dp/B000HNHA12/ref=pd_sim_sbs_e1
Buying tens of these for $200 will be cheaper (and far faster) than a custom solution using the aforementioned SDR (software defined radio).

Though if your goal is a fun project learning lots about FPGAs, PCBs, schematics, RF, and other miscellaneous electrical engineering the SDR options sounds perfect :).

\$\endgroup\$
  • \$\begingroup\$ 6-8 USD with shipping... I can buy up to 30 of them for 200 USD. \$\endgroup\$ – osgx Mar 5 '13 at 20:24
3
\$\begingroup\$

Look into GNU radio. Here's their hardware page to start.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.