The loudest AM radio station on the internet.  Really.
This is an experimental AM broadcast station using a CBS Volumax 4300 *LOUD* audio processor/peak limiter from the 1970's top 40 era (remember WABC, CHUM, CKLW, etc.)

Original 1970s aggressive on-air sound using real electron tube amplifiers, 11kHz preemphasis, Gates Sta-Level audio compressor, CBS VOLUMAX 4300 audio peak limiter.  It doesn't matter how many watts you have, it's the peak to average ratio of the waveform and the time constants used that make the sound!






E-MAIL THE STATION ENGINEER:  borisd _AT_sympatico.ca (type "@" instead of the word _AT_)

500 watt transmitter in operation

Marconi tower antenna
Control room and transmitters at 590/CKEY

If you listened to AM radio in the 1970's, then you know the unique sound it had - from both the programming and technical point of view.  590/CKEY is here for you to listen to today! 

We here at 590/CKEY are proud of using the exact equipment that was used when 590/CKEY was the top station in Toronto in the 1970's.  Our competition was 1050 CHUM with the Drake/PAMS Top 40 format with its tight programming and jingles, and the very high power in the audio sidebands that brought out every watt the transmitter was capable of. 

Competing radio stations scrambled in finding the best equipment for being the loudest sounding station.  An added benefit at the time was the fact that tube amplifiers and transformers were in use in transmitters, which by their very nature having a warm sound responded very well to attempts at driving them harder to produce louder signals even when some distortion appeared.  Attempts today to use multi-band digital audio processing with the transistor switching modulators now used in transmitters, while clean sounding, are not successful in reproducing that classic loud sound, even when playing the exact songs from the 1970's, because they cannot be driven beyond the digital clipping level, which is fixed in absolute voltage level with no headroom to go beyond it, and the multi-band processing has its own artificial artifacts that makes the songs contain a somewhat unnatural frequency distribution - a sound similar to CD's versus vinyl recordings: clean, but lacking something in its character because of the signal approximation as a result of the limited digital resolution 16 bits and low sampling rate 44.1 kHz = artifacts of omission and sampling and aliasing errors, and a coldness due to the entire absence of beating of >20kHz frequency components to lower, audible frequencies, a natural phenomenon of mixing multiple instrumentation harmonics.

There is much confusion about how the classic analog loud sound was achieved, in what order the equipment was connected, and why it sounded the way it did.  Here are the secrets revealed.

In an AM transmitter, unlike an FM transmitter, the power of the carrier is modulated , or added to, by the audio signal.  The more modulation power that is added, the louder the signal, for the same carrier power and antenna radiation pattern.  In theory there is no limit.  In practice there are limits - but there are many tricks that can be used to get around them.  Engineers found these by trial and error, and through experimentation.

Two reasons:

One:  Radio is different than listening to a CD, record, or tape at home.  At home, the environment is quiet, the CD plays directly into an amplifier and loudspeaker system.
Radio uses electromagnetic waves radiating outwards from an antenna...every doubling of distance from the transmitting antenna results in 1/4 the transmitted power available.  There is natural and man made noise present in the atmosphere which the receiving antenna picks up.  There is also noise in the radio receiver.  As the transmitted signal gets weaker, these noises become greater in proportion, ie. the received signal to noise ratio gets poorer.

Two:  A transmitter can only handle a certain peak power output before distorting or clipping or having voltage breakdown.   Natural audio signals have a rather high peak to average ratio.  Turning down the transmitted audio to accomodate the peaks so that they will just drive the transmitter to maximum peak output would mean the average signal would be much weaker...further reducing the transmitted signal to noise ratio discussed in reason one.

The psychology of listening is such that listener fatigue will set in while listening to a poor signal to noise ratio.  The listener will soon tune to another radio station which has a better signal to noise ratio, especially if similar programming is available from another station.  From a marketing point of view, if the transmitted signal to noise ratio can be increased, more listeners will be attracted -- this is the basic premise of the "loudness war" in radio.

Broadcasting stations in north america have a limit of 50,000 watts carrier power output.  The best way to situate the transmitting antenna is outside of a metropolitan area, preferrably near the ocean or a lake, and then use multiple antenna towers to beam the signal in the direction of the metropolitan area (at the expense of other directions).  This will result in the greatest possible transmitted field strength to the major listening area.

After the obvious basic things that can be done to improve signal to noise ratio, transmitter location, power output, and antenna gain, the only thing left is signal processing.

Firstly, the high frequencies can be boosted.  Because most AM radios have IF amplifiers with limited bandwidth, audio above 5kHz is severely attenuated.  The FCC/DOC allows the transmission of up to 11 kHz sidebands, or a total of 22 kHz occupied bandwidth, since adjacent channels are not assigned in the same geographical area. The lack of high frequencies in a received signal in the presence of atmospheric noise in an AM receiver results in a poor signal to noise ratio.  Thus transmitter audio pre-emphasis boosts the high frequencies above the noise level to some degree.  Since the IF amplifier in an AM receiver attenuates high frequencies, including noise, the resultant signal will sound natural again, yet the noise has been reduced.  This gives the impression that the signal is stronger, and also sounds more high fidelity at the same time.  So the signal from the mixing console first passes through a high frequency booster or equalizer.

Secondly, to ensure that the AVERAGE audio level is as high as possible regardless of the program material, a compressor is used.  This compressor must have a medium attack time, around 28 msec, to avoid a sudden loud sound from being obviously suddenly reduced in level.  It must also have a slow decay time > 10 seconds to prevent the average loudness from noticeably changing too quickly, which would cause an unpleasant effect called gain pumping.  All of this ensures that the average audio level is consistently as high as possible without leaving any obvious artifacts that the gain is actually changing.  The Gates Sta-Level is such a compressor, using tubes, and increases the audio level by 2-15 dB compared to not using it.  The actual benefit depends on the program material, the original loudness variations, and the rate of syllabic variation.

Thirdly, in 1970, CBS Laboratories produced a revolutionary product.  The Volumax 4300.  The first analog signal processor for maximum loudness.  It contains two circuits.  A special compressor, and a peak limiter.  This device is connected after the main slow acting compressor.  The special compressor is a FAST acting compressor.  Unlike the main compressor, this one has a medium-fast attack time, around 8.5 msec, and a relatively fast release time, around 260 msec.  Percussive sounds faster than 8 msec do not trigger the compressor and go straight through with their original volume level.  The syllabic sounds are compressed and the gain is re-adjusted very quickly due to the 260 msec.  This only works well if the average signal input is relatively constant - the job of the main slow acting compressor.  The two compressors work together in preventing gain pumping, and allow percussive sounds to sound loud because they don't reduce the gain of the second compressor (ie. their attack time is not altered), yet the syllabic variations are highly compressed and are brought up to maximum gain quickly, making the signal sound very loud, with a further increase of 6 dB or 4x power compared to not using it.

Forthly, because the transmitter uses power supplies and amplifiers that have a limit to their power output capability before clipping, distorting, or causing illegal signal splatter to adjacent channels, the PEAK audio level must be instantly limited to the level before such clipping, distortion, and splatter occurs.  In the case of an AM transmitter, the negative going peaks are critical, since the transmitted carrier cannot go below 0 watts or -100% modulation - if it reaches 0 for any length of time, there is no output at all, and the non-sinusoidal character of such negative overmodulation results in illegal splatter and distortion of the transmitted signal.  The positive peak, on the other hand, is limited only by the power supply and the modulator amplifier capability, as well as the voltage insulation of the transmitter components and can go as high in power as the modulator can cleanly produce without distortion, and as long as voltage breakdown does not occur in the transmitter carrier amplifer.  This can be much more than +100% modulation that a symmetrical AM sine-wave signal is limited to.  Thus unsymmetrical modulation of the carrier allows more sideband power in the signal compared to symmetrical modulation.  A peak limiter is the equipment that is used for this purpose.  It is a very fast acting device, with an attack time < 1 usec.  In this application, only the negative peaks would be clipped to keep them at -100%, while the positive peaks would be allowed to go upwards beyond +100%.  By using the two compressors discussed above, the signal is as loud as can be made before clipping cuts off the excessive peaks - a much easier job to do compared to that if the signal loudness was varying all over the place.  The peaks can be removed very close to the average signal - resulting in full utilization of the volume gain due to compression.

Thus the second circuit in the Volumax 4300 is an unsymmetrical clipping circuit which takes care of the negative peaks, fixing them at the -100% modulation level.  The 6dB improvement from this device alone is all totally useable since at no time will the transmitter be overmodulated in the negative direction, neither will it be undermodulated, as the average signal will always be close to the clipping level.  Every top 40 radio station wanted one of these.  One does not really hear any distortion because the energy in the clipped peaks is very low - and what is left can fully modulate the transmitter toward -100%  almost continuously!

Summary of equipment used in audio chain, in order:

RCA BC-5B tube mixing console

Frequency response 30-15000 Hz +/- 1 dB ref. 1 kHz.  This is the broadcasting standard.

Kahn symmetra-peak, a device that reduces the positive/negative asymmetry on voice signals, to manageable values so that at -100% modulation the positive peaks are under +130% modulation level:  average power gain +2 dB

(Without this device, the highly unsymmetrical nature of voice signals would require the negative direction to be peak limited to less than 100% because the positive peaks would exceed the transmitter components insulation breakdown, which is around +130% and no more.  The alternative is to clip the positive peaks more aggressively = too much distortion.)

Put only in the voice channel -- NOT RECOMMENDED FOR MUSIC (certain bass/percussive wave shapes less steep & weakened (subtle but noticeable) due to non-linear phase rotation ~3.0 msec at 300 Hz, 3.5 mec at 100 Hz, 4 msec at 30 Hz, 0.5msec at 10000Hz )

Linear pre-emphasis of frequencies above 1.2 kHz, reaching +12dB at 11 kHz:  average perceived on-air power gain +3dB

Gates Sta-Level tube compressor, attack time of 28 msec and dual constant release times of 2.35 sec and 10 sec:  average power gain +3dB

Volumax 4300 volume controller/peak limiter
      compressor section, 8.5 msec attack time, 260 msec release time:  average power gain +6 dB
      limiter section, < 1 usec peak limiting of negative peaks for radio transmitter

Radio Transmitter, capable of +130% positive modulation:  average power gain +2 dB (on-air signal only)

Total on-air power gain compared to not using this equipment:   +16dB (or 40x more power - thus a 50 kW station sounds like 2000 kW!)

RCA 44-BX and RCA 77-DX ribbon velocity microphones
ZaraRadio software package mp3/wave file player/jingle&commercial scheduler on HP notebook computer
RCA BC-5B tube broadcast mixer console
Kahn symmetra-peak phase rotator (only in voice channel)
Pre-emphasis from 1.2-11 kHz +1 to +12 dB
Gates Sta-Level tube dual time constant slow compressor
Volumax 4300 fast compressor/negative peak limiter
Brick wall 11 kHz splatter filter
SHOUTcast mp3 streaming audio server on Packard Bell 910C computer, Bell DSL internet line

Since the power level represented by a digital signal over the internet has fixed levels determined by the fixed number of data bits, this energy limited representation requires the positive and negative peaks to be symmetrical +/-100% and to be represented by data bits 0 and 255 for maximum volume.

Thus the peak limiting of positive peaks necessary for the SHOUTcast digital audio service over the internet makes a signal that isn't quite as loud as the on-air signal, but it is still LOUD, +13 dB or 20x more power.  Most internet radio stations use some processing - many use excessive and artificial digital processing resulting in distortion on low frequencies and a listening fatigue due to an unpleasant spectral energy distribution - therefore, we are confident that our signal is the loudest - and clearest - on the internet. 

Please note that the 80 kilobits/second data stream compression will create audible artifacts that would not be heard live on the air.  Plus, some of the "rare" commercials are poor mp3s with artifacts unfortunately...Other than these, the audio is virtually distortionless and clear.

   Radio physicist and engineer.

Scientific Research, Engineering and Manufacturing.
Marconi Wireless...Radio Tubes...Armstrong Regeneration...AM Radio...Superheterodyne...FM Radio


Standard Broadcast Radio owns and operates a radio museum and radio station 590/CKEY AM

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