Wideband recording of historical AM radiofrequency bands

Dear friends of radiomuseum.org,

Imagine that you would like to listen to broadcasting stations with your historical radioreceivers - i.e. on one of the AM bands for LW, MW and/or SW. However, if living in an urban environment you may experience so many RF perturbations that the reception of such stations is practically impossible. Moreover AM broadcasting stations are continuously disappearing so that in future the old devices will remain 'silent'. This is a typical situation for many collectors of ancient radios. It is natural that technological advancement renders old transmission methods obsolete. Nevertheless it would certainly be nice if we could still hear voices and music coming out of the speakers of our valuable old devices. Imagine to play back a record of a complete RF band from the past so that you can even tune through and listen to all stations which were active at the time of recording. Exactly this is made possible by a new project named 'COHIRADIA' hosted by radiouseum.org. An example can be seen in my short demo movie (voice is German, but there are english subtitles).

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.

The story began 16 years ago when I got inspired by a post from Jakob Roschy on 16 Sep 05. He 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 tune again through all stations, thus having an exact 'canned' copy of the AM band at the time of recording - an ingenious idea. Successful attempts by a few RM members encouraged me and I managed to record a series of videotapes with MW and SW broadband signals on a portable Philips VR6711 with acceptable quality from the end of 2006 on. 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 radiofrequency bands.

However, the VR technology has a very limited signal/noise ratio and I could not share the tapes with anybody. By the end of 2020 I 'discovered' the STEMLAB125-14 from RedPitaya for my purpose, i.e. a 10x6 cm 'small' single board computer with FPGA based signal processing hardware. Among other useful applications it can be used as software defined radio and allows to continuously record and playback wideband RF signals with 14 bit resolution. Thus I could digitize all my tapes and store the files on disk. For playback I couple the STEMLAB via a balun to the antenna jack of an AM radio, e.g. a tube device. Personally I tried this successfully on my Ingelen Columbus51, Minerva375, Atwater Kent10 and Eumigette W and I am very happy that Gerald Gauert and Walter Barteczek succesfully acted as 'beta-testers' and played back one of my records on their historical radios after installing their own STEMLABs. Radiomuseum members can read the exciting original report by Gerald Gauert (Post #4) in the (german) main thread of COHIRADIA.

In this context I would like to express my special thanks to Ernst Erb, Ueli Kurmann and Heribert Jung for their invaluable support for the project. Currently COHIRADIA provides a file server and a landing page (English version under preparation), from which the following ressources will be linked:

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

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

Files and data can be downloaded from 3 directories of the fileserver:

• Documentation (versions in both EN and Q)
• Software (with instructions both in EN and G)
• Data (IQ raw files)

Let me mention three selected representative recordings 

• short sample of 30/31-12-2006, New Year's Eve 2006 (4.2 GB)
• New Year's Eve 2021/22, MW, including the final shutdown of Czech AM broadcasting (8.4 GB)
• MW recording made 19-04-2022 in Los Gatos, CA, by Stan Roberts with an SDR (1.7 GB)

In the near future, I will publish the Python sources of my software as an open source project on GITHub so as to make the software available to LINUX users as well. Furthermore, other enthusiasts who are familiar with Python could then write and contribute their own code extensions.

All COHIRADIA data files can also be played back on a normal computer with freely available SDR software such as SDR# from Airspy. For that purpose one only needs to rename the file extension 'dat' to 'raw' and select the file format ci16.

Since early 2021 I have been recording contemporary MW, LW and SW signals directly digitally with an outdoor wire antenna strung in my 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.

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 (price ca. 400$) 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. As an alternative to the STEMLAB recording is also possible with dedicated software defined radios like the ones from Airspy or SDRplay. The recent sample recording made in California by Stan Roberts was taken with a RSP1A by SDRPlay. This record may serve as a pilot example for what COHIRADIA could become: a worldwide mosaic of the AM lanscape before it will disappear and become part of the history of broadcasting.

I guess that, due to the widespread use of SDRs, many international radio amateurs are already in the position to contribute to COHIRADIA. It would be nice to find volunteers from different continents, who are willing to make sample recordings. If you can recommend people whom I should contact, please let me know. And of course I invite all radio collectors who wish to playback original RF spectra with their old AM radios, to download and play back COHIRADIA files with either the STEMLAB125-14 or an appropriate SDR with TX capability.

Best regards

Hermann Scharfetter

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