Wideband recording of historical AM radiofrequency bands

ID: 567631
Wideband recording of historical AM radiofrequency bands 
02.Apr.22 11:04

Hermann Scharfetter (A)
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Hermann Scharfetter

Dear friends of radiomuseum.org,

With a delay of 16 years, I would like to revive a topic that Jakob Roschy had posted on 16 Sep 05, in the thread on tuning through a radio, and which initiated my project COHIRADIA which may be very interesting for some radio amateurs and collectors. The cited thread began in 2004 with Ernst Erb's groundbreaking call to make recordings of the sound when tuning through an AM-radio. Facing the rapid disappearence of broadcasting stations on MW, SW and LW due to decreasing importance of the AM technology, he stimulated the conservation of audio-documents which give listeners a sense of what AM radio "felt like" when it was alive and well - an excellent idea indeed. In a reply to his call Jakob Roschy suggested to try broadband recordings of the entire MW or LW band directly from the antenna by means of a video recorder which by default has a bandwidth of several MHz. When coupling the antenna input of an AM receiver to the video output and playing back the video tape, one can retune all stations, thus having an exact 'canned' copy of the AM band at the time of recording - an ingenious idea.

Some RM members made attempts to do so and their reports encouraged me to try the same. I managed to record a number of videotapes with MW and SW broadband signals on a portable Philips VR6711 with acceptable quality from the end of 2006 on. I captured the entire RF signal with a wire antenna, fed it through a bandpass filter (e.g. 500 - 1700 kHz) to a broadband amplifier and coupled the output to the video input of the VR. Playing back these tapes one can actually tune through and listen again to all stations received at that time. Thus I possess a nice archive of original historical radio signals.

However, VCRs do not have a particularly good signal-to-noise ratio (SNR), which means that weak stations are noisy or even drowned out. There is also a small frequency offset, about +3% on my VR6711, because tracking without a real video signal apparently causes a deviation of the tape speed from the ideal one. Furthermore, I could not copy my tapes without further signal loss and make them available to other radio amateurs, although I wished to share my 'treasures'. The suggestion by Ingo Truppel, to record digitally with 16 bit resolution was out of reach for me at that time due to the lack of a fast digitizer. My dream came true at the end of 2020 with the STEMLAB125-14 from RedPitaya, a 10x6 cm 'small' single board computer with FPGA based signal processing hardware. When connected to a PC the device can be used as oscilloscope, signal generator, network analyzer etc., but also as a software-defined radio (SDR) up to 60MHz. It allows to continuously record and playback wideband RF signals at 14 bit resolution. Thus I could digitize all my tapes and store the files on harddisks. For playback I simply couple the STEMLAB via a balun to the antenna jack of an AM radio, usually a tube device. Personally I tried this successfully on my Ingelen Columbus51, Minerva375, Atwater Kent10 and Eumigette W. For a demo see my video.

Since early 2021 I have been recording contemporary MW, LW and SW signals directly digitally in more or less regular intervals. Running a hobby lab with an outdoor antenna strung in the garden on the outskirts of Graz (Austria), I am in the lucky position of still receiving many southern and eastern European stations as well as a few North African transmitters without experiencing too strong urban interferences. The quality is very satisfying, under good conditions I can even pick up weak stations like Greek 'pirate' transmitters between 1600 and 1700 kHz. During playback on a good MW radio I cannot recognize a noticeable difference between original and recorded signals.

Since several weeks radiomuseum.org is hosting my project COHIRADIA and I would like to express my special thanks to Ernst Erb, Ueli Kurmann and Heribert Jung for their invaluable and excellent support. COHIRADIA stands for CONservation of HIstorical RAdiofrequency bands by DIgital Archiving and aims at collecting and archiving broadband radiofrequency bands from analog sources. The purpose is two-fold:

  • provide interested radio enthusiasts, collectors and museums with original radio signals from the past for playback on historical radio receivers.
  • equip amateurs with tools for making their own recordings of contemporary radiofrequency bands all over the world to build up a versatile and international signal database.

Currently COHIRADIA provides a file server and a landing page, from which the following ressources will be available:

(1) Detailed documentation of the currently available hardware and software

a detailed description for the setup of both hardware and software

(2) An installation package cohiradia_V1.2en.zip (172MB) which contains

  • a detailed description for the setup of both hardware and software
  • a Software for Windows10 (COHIRADIA_RFCorder.exe, beta-version 1.1b)  
  • video tutorials part1 and part 2 which show the practical implementation step by step

(3) a continuously growing collection of data files with recordings for download and playback. In addition, there is a short test file (480 MB) with one and a half minutes from a recording made in 2021. Currently the following data files are available:

I am very happy that Gerald Gauert acted as a first 'beta-tester' and successfully played back one of my records on a radio receiver from 1928 after buying his own STEMLAB - for his really exciting  original report see the german version of this thread.

I would be very happy if soon some more amateurs would buy a Redpitaya-STEMLAB (costs currently ca 350 - 400$) and try out my software. In the near future, I will publish the Python sources of my program as an open source project on GITHub so as to make the software available to LINUX users as well. Furthermore, other enthusiasts who ar familiar with Python can write and contribute their own code extensions.

I would also like to mention another small 'subproject' of COHIRADIA in the section 'Audio', which shows a tuning-through video from a recording of the whole MW band grabbed around midnight December 30/31, 2006. In 'Parallel Clips' the video is linked to 15 extracted audio clips representing 15 annotated stations running exactly synchronously in parallel to the video for ca. 11 minutes. The video also contains German, Swiss and French stations which have ceased their services. Most  tracks contain station announcements and news at midnight, often preceeded or followed by the federal / national anthem. I would like to express my sincere thanks to Heribert Jung  for the tireless help with uploading, revising and linking my clips !

Should viewing/listening to these recordings arouse your interest, then I warmly invite you to participate in COHIRADIA so as to make it a piece of lived history of technology. I would be happy if one or the other enthusiast started also operating a STEMLAB125-14 somewhere else in the world and enjoy the playback on old radios. However, I would be particularly excited if several international amateurs  tried to make own recordings. If people located in different sites worldwide could contribute recordings to radiomuseum.org, then together we could build up a unique and highly diverse supraregional collection of still active AM bands, thus preserving for future generations a small cross-section of what is currently still being broadcast.

Best regards

Hermann Scharfetter


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