Reelin' in the years: Tracking the history of magnetic tape

Part One: From wheels of steel to ribbons of rust


Deliberate bias

The early 20th Century became littered with experiments in sound recording, where magnetic (rather than mechanical) techniques flourished. The recorders were typically built around wire or solid metal tape formats and there are far too many to list here. Yet for a taster, check out the Audio Engineering Society’s historical section which features extracts from the work of Semi Joseph Begun – an innovator in tape recording – whose 1949 book Magnetic Recording: the ups and downs of a pioneer documents the development of the early recorders.

Valdemar Poulsen telegraphon recorder

Valdemar Poulsen telegraphon recorder
Source: Deutsches Museum, München

The relatively low output and issues with the signal to noise ratio rather stilted the acceptance of magnetic recording – although Poulsen's patents in the hands of American Telegraphone Company found favour as dictation machines. The development of valves (vacuum tubes) for amplification and the application of AC bias improved matters enormously. The latter hailed from the US Naval Research Laboratory, being an invention of W.L. Carlson and G.W. Carpenter in 1921 that other inventors around the world would discover or revisit over the following 20 years with Walter Weber being a notable developer of this technique.

AC bias adds an inaudible high frequency (typically from 40kHz to 260kHz) to the audio signal. Its presence helps overcome the inherent resistance magnetic media has to becoming magnetised – a non-linear response – and this allows recording to occur on the more linear region. If you want to brush up on your physics, you can read more about hysteresis loops and coercivity here [PDF].

The extreme usage of AC bias current can either minimise distortion (highest bias) or deliver a broader frequency response (lowest bias). Pro audio gear always errs towards the low distortion option – the old Otari MTR-90 studio multitrack had a bias frequency of 257kHz – whereas consumer products, such as the compact cassette, work on a compromise setting of around 70kHz-105kHz. With magnetising recordings now a pushover thanks to AC bias and amplification able to demonstrate these improvements still further, the remaining area in need of refinement was the media itself.

The BBC's reels of steel

Blattnerphone steel tape recorder

The Blattnerphone: steel tape and DC bias

DC bias had been tried without the same level of success on machines including the Blattnerphone which was to become the BBC’s first tape recorder following trials in September 1930. The corporation was keen to find ways to repeat new broadcasts throughout the day in different time zones for its forthcoming Empire Service. The machine had reels of steel tape 6mm wide and 0.08mm thick that weighed 21lbs and would run at 60ips, clocking up 20 minutes of recording time. This potentially lethal razor tape required careful handling and although the BBC had two in service by 1932, the Blattnerphone was only considered suitable for speech. A home-made 3mm British Blattnerphone appeared but was mechanically noisy and suffered speed fluctuations, despite being more reliable and better built.

So when Marconi bought the rights to the Blattnerphone, the BBC requested a redesign that would feature a direct drive with a swift run-up time, fast rewind and quieter operation. The Marconi-Stille recorder was the result that, according to BBC engineering landmark listings, was bought into service in 1934. It continued to use 3mm steel ribbon yet introduced two tape reservoirs, which in combination with three motors driving feed, speed and reel take-up, facilitated the fast start the BBC needed.

Edits could be performed on the tape – a spot welder was built in – yet increased the chances of breakages. Too many edits could even damage the dual pole heads as the joins in the tape rolled past. The Swedish-made steel tape, containing chromium and tungsten, was hardened, polished and supplied in 1000m drums. The BBC would join them together to create 2700m reels giving 30 minutes of playback time. These were so heavy, weighing in at 15kg, that two people were needed to change them – using a block and tackle was a serious consideration.

Marconi-Stille steel tape recorder

Marconi-Stille steel tape recorder

The aim was to produce a recorder with a 45dB signal to noise ratio (SNR), but as the BBC’s own R&D report from 1937 shows, getting figures of a 35dB SNR appeared to be a good achievement, along with a frequency response of 50Hz to 5kHz, ±2dB.

The broadcaster continued to use the Marconi-Stille recorder into the 1950s but given its double speed rewind, that spooled this razor ribbon at 120ips, these wheels of steel were kept in machine rooms for self evident safety reasons. For a fascinating exploration of these recorders then look no futher than Roger Beckwith's Old BBC Radio Broadcasting Equipment and Memories site.

Next page: Rust and tone

Similar topics


Other stories you might like

  • FTC urged to protect data privacy of women visiting abortion clinics
    As Supreme Court set to overturn Roe v Wade, safeguards on location info now more vital than ever

    Democrat senators have urged America's Federal Trade Commission to do something to protect the privacy of women after it emerged details of visits to abortion clinics were being sold by data brokers.

    Women's healthcare is an especially thorny issue right now after the Supreme Court voted in a leaked draft majority opinion to overturn Roe v Wade, a landmark ruling that declared women's rights to have an abortion are protected by the Fourteenth Amendment of the US Constitution.

    If the nation's top judges indeed vote to strike down that 1973 decision, individual states, at least, can set their own laws governing women's reproductive rights. Thirteen states already have so-called "trigger laws" in place prohibiting abortions – mostly with exceptions in certain conditions, such as if the pregnancy or childbirth endangers the mother's life – that will go into effect if Roe v Wade is torn up. People living in those states would, in theory, have to travel to another state where abortion is legal to carry out the procedure lawfully, although laws are also planned to ban that.

    Continue reading
  • Zuckerberg sued for alleged role in Cambridge Analytica data-slurp scandal
    I can prove CEO was 'personally involved in Facebook’s failure to protect privacy', DC AG insists

    Cambridge Analytica is back to haunt Mark Zuckerberg: Washington DC's Attorney General filed a lawsuit today directly accusing the Meta CEO of personal involvement in the abuses that led to the data-slurping scandal. 

    DC AG Karl Racine filed [PDF] the civil suit on Monday morning, saying his office's investigations found ample evidence Zuck could be held responsible for that 2018 cluster-fsck. For those who've put it out of mind, UK-based Cambridge Analytica harvested tens of millions of people's info via a third-party Facebook app, revealing a – at best – somewhat slipshod handling of netizens' privacy by the US tech giant.

    That year, Racine sued Facebook, claiming the social network was well aware of the analytics firm's antics yet failed to do anything meaningful until the data harvesting was covered by mainstream media. Facebook repeatedly stymied document production attempts, Racine claimed, and the paperwork it eventually handed over painted a trail he said led directly to Zuck. 

    Continue reading
  • Florida's content-moderation law kept on ice, likely unconstitutional, court says
    So cool you're into free speech because that includes taking down misinformation

    While the US Supreme Court considers an emergency petition to reinstate a preliminary injunction against Texas' social media law HB 20, the US Eleventh Circuit Court of Appeals on Monday partially upheld a similar injunction against Florida's social media law, SB 7072.

    Both Florida and Texas last year passed laws that impose content moderation restrictions, editorial disclosure obligations, and user-data access requirements on large online social networks. The Republican governors of both states justified the laws by claiming that social media sites have been trying to censor conservative voices, an allegation that has not been supported by evidence.

    Multiple studies addressing this issue say right-wing folk aren't being censored. They have found that social media sites try to take down or block misinformation, which researchers say is more common from right-leaning sources.

    Continue reading
  • US-APAC trade deal leaves out Taiwan, military defense not ruled out
    All fun and games until the chip factories are in the crosshairs

    US President Joe Biden has heralded an Indo-Pacific trade deal signed by several nations that do not include Taiwan. At the same time, Biden warned China that America would help defend Taiwan from attack; it is home to a critical slice of the global chip industry, after all. 

    The agreement, known as the Indo-Pacific Economic Framework (IPEF), is still in its infancy, with today's announcement enabling the United States and the other 12 participating countries to begin negotiating "rules of the road that ensure [US businesses] can compete in the Indo-Pacific," the White House said. 

    Along with America, other IPEF signatories are Australia, Brunei, India, Indonesia, Japan, South Korea, Malaysia, New Zealand, the Philippines, Singapore, Thailand and Vietnam. Combined, the White House said, the 13 countries participating in the IPEF make up 40 percent of the global economy. 

    Continue reading
  • 381,000-plus Kubernetes API servers 'exposed to internet'
    Firewall isn't a made-up word from the Hackers movie, people

    A large number of servers running the Kubernetes API have been left exposed to the internet, which is not great: they're potentially vulnerable to abuse.

    Nonprofit security organization The Shadowserver Foundation recently scanned 454,729 systems hosting the popular open-source platform for managing and orchestrating containers, finding that more than 381,645 – or about 84 percent – are accessible via the internet to varying degrees thus providing a cracked door into a corporate network.

    "While this does not mean that these instances are fully open or vulnerable to an attack, it is likely that this level of access was not intended and these instances are an unnecessarily exposed attack surface," Shadowserver's team stressed in a write-up. "They also allow for information leakage on version and build."

    Continue reading
  • A peek into Gigabyte's GPU Arm for AI, HPC shops
    High-performance platform choices are going beyond the ubiquitous x86 standard

    Arm-based servers continue to gain momentum with Gigabyte Technology introducing a system based on Ampere's Altra processors paired with Nvidia A100 GPUs, aimed at demanding workloads such as AI training and high-performance compute (HPC) applications.

    The G492-PD0 runs either an Ampere Altra or Altra Max processor, the latter delivering 128 64-bit cores that are compatible with the Armv8.2 architecture.

    It supports 16 DDR4 DIMM slots, which would be enough space for up to 4TB of memory if all slots were filled with 256GB memory modules. The chassis also has space for no fewer than eight Nvidia A100 GPUs, which would make for a costly but very powerful system for those workloads that benefit from GPU acceleration.

    Continue reading

Biting the hand that feeds IT © 1998–2022