Audio Bit Rates and Formats

Occasional reader and blogger Robert has broken down all the information on audio bit rates and audio formats.

His work can be found here: Ultimate Guide to Audio Bit Rates and Formats

It is a good read, especially for those that use audio streaming as their main content distribution method.

Streaming only stations used to be a big thing but have been supplanted by Spotify and Pandora.  I am not a huge fan of either of those services but I do like to listen to podcasts.

Good audio should be near the top of the list for any content provider.  Few things are more annoying than listening to an interesting podcast with low volume, background noise or other technical defects.

Hearing test

This was on the Wheatstone news letter a few months ago. NPR has an interesting test to see if one can hear the difference between various quality .mp3 and .wav files.  There are six cuts with three versions each; a 128 kbps .mp3, a 256 kbps .mp3 and a .wav file in no particular order.

That NPR article can be found here: How Well Can You Hear Audio Quality?

I listened to all of them and found the 128 kbps .mp3 was pretty easy to pick out.  On the newer material, it was sometimes difficult to tell the difference between the .wav file and the 256 kbps .mp3.  Keep in mind that most radio stations stream at 64-128 kbps.  Online music services like Pandora (64 kbps for free listeners, 192 kbps for subscribers), Spotify (96-160 kbps for free listeners, 320 kbps for subscribers) and Apple (256 kbps for everybody) offer slightly better quality, especially for paid subscribers.

It is too bad one cannot simulate 15 IPS analog tape.  I would bet that a well mastered recording on analog tape would stand out above anything even remotely compressed.

Others have compared streaming audio to analog FM audio and found that fairly high bit rates are needed to make the quality equal: What bitrate is needed to sound like analog FM?

One more thing to keep in mind, HD Radio runs 96-144 kbps on the main channel and 20-60 kbps on the sub channels.

My appologies for the lack of posts

Two reasons for this; first, I am deep into the IP networking curriculum and time is at a premium.  That being said, I am rather enjoying myself in school, which is always good.  Secondly, and related to the first part, I have not been spending too much time these days doing Broadcast Engineering work.  Thus, the subject matter and various topics have not been jumping out at me as they normally do.

My busy schedule not withstanding, there are some interesting things going on in the realm of Radio Engineering:

  1. On the LPFM front, the FCC has dismissed over 3,000 translator applications from the great translator invasion of 2003.  This is great news and now potential LPFM applicants can use the FCC LPFM search tool to get a good idea of what is available in their neck of the woods.  Other search tools include Recnet and Prometheus Radio project.  Filing window is October 15, 2013, apply now or forever hold  your peace.
  2. Chris Imlay has some good ideas on AM revitalization. His suggestion is to have the FCC enforce and strengthen its existing rules regarding electrical interference. I notice two letters are missing from his list, those would be “h” and “d.” While the ideas are technically sound, it seems unlikely that the FCC can or would be able to enforce stricter Part 18 rules.
  3. Lots of EAS shenanigans going on with zombie alerts and hijacked EAS systems.  Really people, default passwords?  Secure your equipment and networks or pay the price for complacency.  Nearly all new equipment has some sort of web interface, which can be a great time saver.  They can also be easily exploited if left vulnerable.  Fortunately, this was not as bad as it could have been.
  4. Something happened in NYC that hasn’t happened in quite a while.  Country music filled the air on a station that is generally receivable in the five boroughs.  This may not seem like big news to the rest of the country, but in market number one, it is big news.  Further, Cumulus has registered “NashFMxxxx.com” for every FM dial position.  National country channel in the works?  I’d bet yes.  A look at recent trends shows that Cumulus is standardizing formats on many of its AM and FM stations, making them, effectively, part of a nation network of over the air repeaters.
  5. Clear Channel has put more effort into iHeartradio, for seemly many of the same reasons as Cumulus’s standardized formats.

Where is this all going?  There are several trends evident including; AM will eventually be declared DOA and switched off, transition to national based music formats, an emphasis on IP (internet) based delivery systems, an eventual phase out of local programming, smaller staffs concentrated on local sales and little else.

The single bright spot could be LPFM.  Only time will tell if this new crop of LPFM licensees will keep the faith and tradition of local radio.  If one looks at the natural course of evolution, under times of extreme stress, species tend to get physically smaller in response.  The larger species cannot sustain themselves with the necessary energy intake and die off.  See also: Dinosaurs.  I certainly would call this prolonged, nearly dead economy stressful on the broadcasting business.  Perhaps, when all is said and done, it will be the small, volunteer LPFM still on the air and serving the community.

Stereo Tool

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
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.