Engineering Radio

FM and AM broadcast radio processing has gone through many iterations.  At first, the main processing function was to limit the input audio to a transmitter and prevent over modulation.  This was a particular problem with early tube type AM transmitters, where over modulation could create power supply overloads and kill the carrier while engineers scrambled around resetting things and hopefully pressing various buttons to get the transmitter back on the air.

Over the years, processors incorporated not just limiting, but compression, gating, equalization, clipping and so on all in an effort to keep ahead or at least abreast of the station across town.

Today, broadcast air chain processors come in all shapes and flavors.  In addition to that, internet streaming stations have their own unique set of issues to deal with.  The top of the line Telos Omina or Orban Optomod systems are great, however, they can set one back a pretty large sum of money.  Enter then, the Stereo Tool PC based software processing program.

Stereo Tool sofware screen shot

The first difference between, say the Omina and Stereo Tool is the end user decides the hardware and basic operating system.  The second difference is Stereo Tool comes with a free trial.  Then there is the price difference, which ranges from about $48.00 US for the basic version, to $161.00 US for the basic FM version and finally $269.00 US for the full version (actual prices are in Euros, which will fluctuate day to day and the credit card company will likely charge an exchange fee).  Add to that a medium speed (2 Ghz) Intel Pentium4 or better computer, 1 Gb or more of RAM, good sound card and it all comes out to a reasonably priced audio processor.

Here are some of the specific features for broadcasting:

• Declipper
• Hiss Removal Filter
• FM Hiss Removal Filter
• Automatic Gain Control (AGC)
• 10-band multiband compressor / limiter / clipper
• Phasing error (AZIMUTH) correction filter
• Stereo booster
• Bass booster
• Final limiter
• Distortion masking Loudness filter
• Lowpass filter
• FM pre-emphasis filter
• FM stereo encoder
• FM RDS encoder
• Composite limiter

Much more info at the Stereo Tool website.

The idea of PC based audio processing is new and interesting to most of us.  The designer and owner of Stereo Tool, Hans van Zutphen, was nice enough to answer a few questions I posed to him via email:

PT: What prompted you to write audio processing software?

HvZ: Since I was very little I’ve always wanted to have my own radio station. I remember playing with walkie-talkies and trying to receive their sound on a real radio when I was about 8 or 9. I never really did anything with it until I found out in 2001 that you could easily start a webradio station – I actually found out because I was listening to a pirate station in my car which turned out to have a stream; within a week my own station was online.

Of course I needed a bit of processing for it, and I wrote some command line tools – a singleband compressor, a stereo to mono convertor that didn’t cause any loss of audio (I was broadcasting hard trance on a mono 56 kbit/s stream, and this was the only way to get a decent sound out of it), and some time later a multiband compressor.

In 2004 I left the company I worked for (ASML, they make machines to make computer chips, customers are companies like Intel, AMD etc.) to start working for Philips Healthcare, where I was going to work on image processing for X-Ray systems. I had 2 months of ‘spare time’ between those jobs, and I wanted to learn to program in Visual C++, so I decided to a GUI around my command line tools, and make a Winamp plugin out of it. I called it ‘Radio Tool’. I never really planned to do anything with it, it was just an exercise project.

About a year later I came across the Winamp site again and I saw that you could upload plugins. So I uploaded my program, now renamed to ‘Stereo Tool’ because a Google search for “Radio Tool” gave far too many hits. Within a week there were over 1000 downloads and a while later it surpassed 90,000. At that point I decided to create a new version, Stereo Tool 2.0.

For quite a while this remained a hobby project, I occasionally worked on it for a few months and then I wouldn’t look at it for months. But at some point I was approached by someone people who worked at a “real” (FM) Dutch radio stations who asked for some extra features – he couldn’t get the audio loud enough, and that’s how I got into clipping. Things started to get better, I learned more and more about processing, the number of downloads increased and people became more and more enthusiastic about it. At some point, after reading something about how an FM stereo signal looks, I thought it might be possible to output a stereo signal with a 192 kHz sound card, so I bought one and did some tests and it worked that same night, and within a few weeks I added RDS.

PT: Do you know, approximately, how many stations (AM/FM/internet) Stereo Tool is being used on?

HvZ: FM: About 500, ranging from small community and pirate stations up to large nation-wide stations which run Stereo Tool at a dozen transmitter sites. Streaming: Not sure, but definitely over 1,000, probably a lot more.

PT: I have read through the forums on your site, Stereo Tool looks like a very complete processing system. Any plans for new features, future upgrades, etc?

HvZ: Yes. I’m currently working on a new multiband compressor. The multiband compressor in Stereo Tool is still based on the code that I wrote in 2001 for my webradio station, which in turn was based on an even older version that I had used on 8-bit audio. It also has far too many bands. Because of this, the multiband compressor is currently the weak spot of Stereo Tool. In the last weeks I have made a new singleband compressor that sound a lot better, it actually outperforms other compressors I have tested, and I expect great results for the new multiband compressor, which will also have less bands. Something else that I’ve been planning for a long time is a composite clipper, which will add 1-2 dB of extra loudness and especially better highs. Stereo Tool can already be louder with good audio quality than nearly any hardware box on the market (see for example this video, Radio 538 uses an Orban 8600 http://www.youtube.com/watch?v=4VpfcqUPQys – unfortunately due to the mpeg compression it’s a bit difficult to compare but listen for distortion ) – but there’s always room for improvement.

PT: What are the advantages of a PC software based processor vs. a hardware based (e.g. Omni or Optomod)?

HvZ: Ah, good question. Not sure if it’s the right question… With processing, a lot of things come down to taste, and there are several stations that have replaced their hardware processing by Stereo Tool not because it’s software and PC based but because they preferred the audio that comes out of it. Stereo Tool is also one of only 2 processors that contain a declipper (the other one is the Omnia 9, I licensed my declipper to them). For a demo of the declipper see: http://www.youtube.com/watch?v=oqOljvx9KaM

Also, Stereo Tool contains a stereo and RDS coder, most other processors don’t, so instead of having a whole bunch of devices everything can be done in a single PC, which also results in a better quality. Recently I added a new feature that enables synchronizing multiple FM transmitter signals that all connect to a simple Shoutcast stream (video: http://www.youtube.com/watch?v=GYQ5CYs0ZX8 ), so you also don’t need any streaming hardware anymore.

Of course there’s the price. A hardware box that gives “similar” quality (of course every processor sounds different, and it’s a matter of taste, so it’s difficult to compare, but I’m assuming that things like low volume levels, gain riding, distortion and lack of clarity in the highs are bad) easily costs $10,000 or more. And you can always easily upgrade to new versions. If you already have a PC with enough spare processing power you don’t need to buy anything.

I know that some people at radio stations are ‘afraid’ of using a PC in their processing path, but based on feedback I get from the stations that run my software it’s completely stable – and of course if a PC does break, you can replace it with any fast enough PC you have lying around – you just need to put the proper sound card in.

But for development, the advantages are huge. If you use DSP’s, it’s usually a lot of work to even make a very small change. When I worked at Philips Healthcare, the image processing that had been done – without much changes – on DSP’s for many years was being converted to PC’s because of speed of development and price of hardware. Once the conversion was finished, the development speed increased dramatically and 2 years later the image quality had improved beyond anything that was imaginable with DSP’s. PC’s get faster every year, and you don’t have to do anything for that – for the same price the processing power that you can buy roughly doubles every 1.5 years, and if you pay more you can get even more. If you use DSP’s, you have to do a lot of work yourself, you cannot just ‘buy a faster DSP’. Testing things is very easy, I can write some code that does something new, post it on my forum and I’ll have feedback from users the next morning – with DSP’s that’s a LOT more difficult and it takes a lot more time. I’ve learned by now that everyone hears things in a different way, and occasionally there are groups of people who hear something they find very annoying while many other people (often including myself) don’t hear anything wrong with it at all. Especially in cases like this it’s really great to be able to quickly send new versions to several people all around the world for testing.

PT: Are there any particular sound cards that work better with Stereo Tool?

HvZ: Yes. For the best results, use the Marian Trace Alpha, with the ESI Juli@ as second-best choice (it needs calibration).

Thank you very much, Hans, for the interesting insight.

Checkout the videos, especially the declipper video, which is quite amazing.  That will cleanup all but the most ham handed DJ mistakes.

PC based audio processing software is a great solution stations on a limited budget that cannot afford high end air chain processors.  There are many LPFM’s, Part 15 stations and others that can get great sounding audio and RDS for a very reasonable price.  Currently, the AM settings do not allow asymmetrical modulation, which is more of a US thing.  There is some talk of adding it in a later update.