September 1944 was not the ideal time to be making music in Berlin. The Allies had landed on the French beaches, had liberated Paris and were closing in on the German capital. Allied bombers were pounding the city and Hitler had given the order to launch the first V-2 rockets at London. Although no one outside Germany knew it, Hitler was already using another secret weapon 鈥� the first hi-fi tape recorder 鈥� to make the Allies think he was a target for bombing raids in one city, when really he was safe in another.
In the midst of this maelstrom, conductor Herbert von Karajan was holed up in a Berlin radio studio, using Hitler鈥檚 secret tape system to make experimental hi-fi stereo recordings of a German orchestra that still sound stunning today.
NO ONE has ever been able to agree on who invented television. As the 50th anniversary of home stereo approaches, it will quickly become clear that no one can agree who invented stereo sound recording, either. One thing is certain, though. The names Helmut Kruger and Ludwig Heck are seldom mentioned, although the stereo recordings they made in war-torn Berlin 60 years ago sound remarkable even by today鈥檚 audio standards.
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The story begins in peacetime, at the Funkausstellung or Radio Show in 1935. Held in Berlin every two years since 1924, the show has traditionally been the launch pad for new electronic ideas. In 1935, the big news was the Magnetophon, a tape recorder made by AEG Telefunken, with plastic tape from the BASF division of German chemical giant IG Farben.
The idea of magnetic recording was already old. Watch the 1934 British movie Death at Broadcasting House and you will see the murderer exposed by an accidental recording made by BBC engineers on giant reels of rapidly moving metal strip. But the idea of capturing sound on thin plastic tape coated with magnetic oxide was completely new.
The Magnetophon鈥檚 sound quality was poor, though. The constant direct-current 鈥渂ias鈥� signal needed to make the magnetic particles more sensitive to weak microphone signals blanketed the recorded sound with hiss. Getting rid of the hiss, by muffling the high frequencies, made the music sound dull and lifeless. The effect was all too evident on recordings made by BASF engineers when Thomas Beecham and the London Philharmonic Orchestra gave a concert at the music hall in BASF鈥檚 headquarters at Ludwigshafen in November 1936.
All that changed in 1940, thanks to a lucky accident in a Berlin lab. Walter Weber and his boss Hans Joachim von Braunmuhl were engineers at the RRG 鈥� the German equivalent of the BBC (the Reichs Rundfunk Gesellschaft, later renamed Sender Freies Berlin or SFB). One day they were tweaking the circuitry of an AEG tape recorder in an attempt to improve the sound quality, and they adjusted one control too far. The circuitry went into positive feedback 鈥� like the so-called howl round heard when a microphone is feeding sound signals to a loudspeaker, which then feeds back the sound to the microphone, and so on. The tape recorder circuit oscillated at several tens of kilohertz, producing whistle tones too high-pitched to hear. But the signals shook up the magnetic particles on the tape, making it super-sensitive to weak microphone signals.
The AEG machine was suddenly able to record sound with a quality very close to today鈥檚 FM radio and better than most MP3 players. Von Braunmuhl and Weber filed for patents, AEG bought the rights and hi-fi tape recording was born.
The head of AEG said late in 1940: 鈥淭he use of high-frequency bias has opened up for us the possibility of making the tape recorder, with one blow, into an acoustically outstanding instrument.鈥� How right he was. All tape recorders have used high-frequency (HF) bias ever since.
The breakthrough was good news for Adolf Hitler. From the early 1940s he was able to record a speech on tape in one city, for playback and transmission in another part of Germany. The Allied engineers who continually monitored Germany鈥檚 broadcasts in the hope of bombing Hitler mid-speech were fooled because there was none of the hiss expected from tape, nor any of the cyclic noise of disc recordings. As far as they were concerned Hitler was there in the studio, speaking live to the nation.
The Allied engineers were even more puzzled when in 1943 the RRG began to broadcast live orchestral concerts in the middle of the night. One of those engineers was John Mullin, a liaison officer with the US Signal Corps who was stationed in the UK ahead of D-Day. He realised there were no musicians in the studio: what he was hearing was recorded music captured by a new recording system far better than anything previously available. But he had no idea what it was and how it would change his life.
Berlin鈥檚 radio station was based at the Haus des Rundfunks, or Broadcasting House, with a large hall that was used for broadcasting live symphony concerts. RRG had started tape-recording music in the hall in 1942 for broadcast from transmitters around the country, and the following year sound engineers Ludwig Heck and Helmut Kruger experimented with one of the HF bias Magnetophons. They added a second magnetic head so that two tracks could be recorded in parallel.
Kruger then made test recordings of concerts in stereo without telling the musicians. He hung three microphones a metre above the orchestra, one to the left of the stage to catch the violins, one on the right for the basses, and one in the centre, 2 metres behind the conductor.
The modified Magnetophon ran the tape at 77 centimetres per second to get the best possible quality. The sound from the side microphones went to the two separate recording heads, and the central sound was spread evenly between the two channels 鈥� in much the same way that concert broadcasts are recorded today.
鈥淏y the end of the war, we had about 200 to 300 excellent stereo recordings which were stored here in the bunker,鈥� Kruger recalled in 1993 when the international Audio Engineering Society honoured him at a convention in Berlin.
In 1945, at the end of the war, most of the RRG sound archive vanished when the Russians took over the building.
But five stereo recordings survived. Two came from the Polish town of Koscian, where the German army had built a military hospital for war invalids. The RRG laboratories had moved there too. The lab director was von Braunmuhl and he organised concert evenings for the hospital doctors at which he played the latest recordings from Berlin on a stereo Magnetophon.
At the end of the war, two of these tapes 鈥� including an extraordinarily high-quality stereo recording of Anton Bruckner鈥檚 Symphony No. 8 in C minor played by the Orchester der Berliner Staatsoper under Herbert von Karajan on 29 September 1944 鈥� were shipped back to Berlin in private luggage and given to Kruger.
Three more stereo tapes were later found in Moscow. A Russian officer had given them to the state archives, along with 1500 mono tapes. In 1991, after the Berlin Wall fell and the Soviet Union broke up, the tapes were returned to the RRG鈥檚 successor, the SFB.
Engineers from the SFB and the Audio Engineering Society copied the original analogue recordings to digital tape and dubbed excerpts to CD. No electronic doctoring whatsoever was used. The sound quality, especially of the September 1944 tapes, never ceases to amaze. Sadly, copyright constraints have so far made commercial sale of the dubs impossible.
That is not quite the whole story, though. In 1945, Mullin was sent to Frankfurt to investigate tales of a 鈥渄eath ray鈥� station built by the German army to bring down Allied aircraft with high-power radio signals that disrupted their ignition electrics. While there, he visited the local radio station and found several HF bias Magnetophons with BASF tape. Suddenly he understood how he had heard broadcasts of recordings that sounded live.
Mullin shipped the recorders back to the US, and with financial backing from Bing Crosby, who wrote him a cheque for $50,000, he worked with US electronics company Ampex and chemical company 3M to recreate the German technology. Crosby started pre-recording his radio shows in 1947.
Ironically, because Mullin took his Magnetophons from Frankfurt rather than Berlin, they were mono machines. The world had to wait until 1949 for American and British engineers to catch up with Germany and tape-record in stereo.