2010-04-24
Sports Ears Broadcast Receivers
This Saturday I went to see the Sydney Swans slaughter the West Coast Eagles at the Sydney Cricket Ground. Murray Tregonning & Associates has been providing the Sports Ears stadium narrow casting service and associated receivers for other football codes for some time now, but this week was the first time they have offered it at AFL games in Sydney. The general idea is to broadcast the game officials chatter to listeners present in the stadium. My partner is a huge AFL fan and just had to partake in the new service, so she parted with $45 of her hard earned cash for one of their receivers at the game. I instead took my Yaesu VR-500 (a considerably more expensive device) with me to find the transmission and study the system in more detail.
The Magic Frequency
It took only a few minutes to locate the appropriate frequency. The transmission is wide-band FM at 70.2 MHz. The signal strength is reasonable throughout the stadium, probably several watts of transmitter power located locally. Polarisation is difficult to determine with all the scattering of the structure of the stands and people's bodies. Multi-pathing can at times cause deep fading as people move about in proximity, but this is not particular to the system, just a natural consequence of RF communication at this wavelength - FM broadcast band reception suffers similarly.
The Sports Ears® Receiver
The AFL Sports Ears receiver itself is a fairly conventional AM/FM broadcast receiver with a 3rd "UMPS" position on the band switch. In the UMPS position the local oscillator is fixed-tuned and receives the 70.2 MHz transmission regardless of the tuning dial position. The AM and FM positions operate normally, tuning 540-1600 kHz and 88-108 MHz respectively. The FM reception is monophonic in either UMPS or FM.
Inside the plastic enclosure is a small hybrid through-hole and SMD board implementing the receiver. The CD1691CB features prominently - a single largish SMD-package AM/FM broadcast receiver and AF power amplifier chip. The AM and FM circuits are very conventional, almost the reference design in the datasheet. The UMPS channel utilises a crystal locked local oscillator, operating at 59.5 MHz for low-side injection (10.7 MHz IF with your typical ceramic filters and quadrature coil).
The UMPS crystal is in HC-49/U format, and is completely unmarked - I have no idea if this is an attempt to hide its frequency or simply the result of a custom-manufacturing run. The crystal is one of the larger components of the receiver, and is mounted on the "back side" of the board, accessing it requires removal of three screws and carefully unthreading the red plastic tuning dial indicator from its track. It is no doubt an overtone crystal, but I did not unsolder it to measure its properties directly.
The local oscillator leakage is easily detected at quite some distance from the receiver - even my wavemeter can register the LO energy - but no significant sub-harmonic radiation is detectable.
The image frequency 48.8 MHz is not very suppressed, the image response on the FM broadcast band appears somewhat better, as does its sensitivity, but this was not assessed with much precision. Like most radios of this type the antenna for VHF is the headphone lead and controlling the applied signal is a little challenging without modifying the circuit. On FM and AM the LO injection is high-side (the AM IF in 455 kHz), the choice of low-side injection for UMPS is probably to place the image further from the FM broadcast band. The TV sound carriers for analogue TV channel 2 (69.75 MHz) are quite close to the 70.2 MHz UMPS channel and the receiver's selectivity is barely adequate in strong-signal areas when the narrow-casting signal is absent.
Physically the unit is a nice pocket/palm compatible size. It comes with a lanyard featuring a quick-release connector, and a pair of ear-bud earphones of surprisingly acceptable quality. The earbuds are hard-plastic and come with no textile covering "socks" like some other ear-bud style phones do, in particular I found they can slip out of larger ears easily because of the fairly low friction offered by the hard plastic against skin. On the other end of the anatomy scale, smaller-eared individuals may find them too large for comfort. The earphone socket is a conventional 3.5 mm stereo one and phones of your choice are easily substituted. Sports Ears themselves offer over-the-ear padded headphones as an accessory for $20, I can't comment on their quality. The audio power available from the unit before unacceptable distortion is quite adequate with the headphones supplied, even in the noisy stadium environment.
The AFL unit has one rather annoying feature, the tuning dial pointer is coloured red, as is the dial background, making it almost impossible to see even in good lighting conditions.
I took the opportunity when I had the receiver dismantled to colour the pointer black with a felt-tip marker so as to provide some contrast to the display. This is likely an accidental oversight of rushing the device into production, the red colour scheme of the casing being very similar to the pointer. Other code's units have better contrasting colour schemes. Unlike the higher priced units offered for Rugby, the AFL unit has only one receiver and can not simultaneously offer both the umpire audio and FM or AM broadcast band commentary.
The unit is powered by two AAA-size batteries, user replaceable and supplied in the initial package. Current consumption is about 14 mA on FM and UMPS, and 12 mA on AM, at peak AF output power it can rise by an additional 3-4 mA. I strongly suspect the bulk of the current is being consumed by the 3 mm red LED power indicator light, but I did not disconnect it to make a comparative measurement. The cells provided with my partner's unit are brand-name (Mitsubishi) alkaline. Assuming a capacity of about 1 ampere-hour they should run the receiver for about 65 hours. The unit is marketed as having sufficient battery capacity for the entire AFL season, this is quite accurate assuming it is used only 2-3 hours per game. Having the power LED to indicate the "on" state likely helps prevent accidental battery depletion when not in use, despite undoubtedly using quite a large amount of energy itself.
Value?
Their fine marketing aside, is the Sports Ears product value for money? At $45 it is extremely expensive for what's inside the box. A typical AM/FM broadcast receiver of similar quality would be in the vicinity of $10 retail and quite a bit less in large quantity. The addition of the special event channel reception is clearly a fairly custom feature, and some development has gone into adding this feature to an otherwise widely used general "chassis" (not unlike the hundreds of similar merchandising receivers you can get made in Asia in any number of different packaging).
Some unspecified percentage of the sales of the units are contributed back to the AFL and distributed to the clubs. No doubt this particular marketing point is important to many buyers, but a little too weakly specified for my liking.
The entire market for these devices is based on the transmission being on a frequency outside the usual tuning range of widely available receivers. Special Event FM radio licences are available from ACMA, it would have been just as technically easy to offer the service on an FM broadcast frequency that required no special equipment to receive. This is clearly more about creating a captive market for merchandise than simply providing a "fan-based" adjunct to the at-game experience.
There is of course nothing fundamentally wrong with this approach, it encourages attendance to the games, especially if the game official audio is not provided on other coverage of the event. How popular it will be, especially as knowledge of the choices made in its implementation become public remains to be seen.
Personally I found listening to the umpires very instructive and believe it does add a lot to the experience. I frequently record the pay-TV coverage of the event for later viewing, even when I attend the event live - mainly so I can hear the commentary and umpire audio explaining the decisions that can otherwise be quite perplexing when seen live without the benefit of hearing the calls.
The quality of the audio is quite high. Too high at times perhaps, some gentle souls may find the gasping, swallowing and expectorating sounds a little disturbing, although the pre-game and quarter break advertisement loop (and online marketing) warns about the uncensored nature of the content, especially concerning profanity. Further scanning around during the day suggests the microphones carried by the umpires operate around 225 MHz and likely use a diversity reception system to eliminate fading. Other interesting SCG specific frequencies are around 499 MHz, although my VR-500 had insufficient selectivity to separate some of the powerful concurrent transmissions in close proximity, especially the camera direction instruction transmissions.
Security Through Obscurity?
A junk-store FM radio costing only a few dollars can be trivially modified by a suitably handy person to receive the 70.2 MHz transmission. No doubt the great majority interested in listening in will just cough-up the $45 for a Sports Ear receiver, despite the fact the transmission is unencrypted and emitted not too far from the FM broadcast band so as to make modification of a conventional receiver quite practical (and economical production of the Sports Ears receivers for that matter).
The controlled market for the receivers is obviously bad for the fans. Especially considering the current pricing point. I can't see how Sports Ears could prevent other agents developing receivers for the frequencies used in AU and NZ. I assume their only protection is the expense of development and barriers to penetration of the market. The market is surely large enough, but no doubt competing receivers would not be sold at the games in the merchandise stalls.
No mention of the frequencies involved are made on the product sites. In fact a search of the ACMA registry of radio communication licences found no entry for 70.2 MHz. Murray Tregonning as an entity is well known to the ACMA, but hold no current licences at all. I could find no entries for anything related to AFL, SCG or Sports Ears either. How the frequency can go unspecified in the ACMA database is unclear.
I have no idea if one of the NRL and ARU frequencies are the same as the AFL one, but it would not surprise me if they were be shared. The Rugby broadcasts clearly use at least two different frequencies. The NZ frequencies are different it seems, probably because of country-specific licensing. In any case a receiver capable of WBFM reception would quickly find them, probably not too far away in the VHF-L band.
Making Your Own Compatible Receiver
Retuning a cheap FM broadcast receiver to cover the 70.2 MHz band is actually very easy. Almost any non-digital tuning FM broadcast receiver will do, but a turn-the-dial rather than push-to-scan receiver is probably a safer bet to try. Simply locate the local oscillator coil and squash its turns together to move the frequency down. In many cases there will be sufficient range of adjustment to do this, otherwise you may need to add additional capacitance across the oscillator coil, a trimmer can be soldered across it and set to place 70.2 somewhere in the range of tuning. You can use the channel 2 TV audio signal as a reference, if you can receive it at the bottom of the tuning range (and perhaps still get the bottom couple of the FM radio stations now at the top-end of the dial), then 70.2 MHz will be just above, quite close to the TV audio.
FM radios designed for the Japanese market are even closer to the frequency in question and need less prodding and poking to get aligned.
Alternatively if you don't feel confident hacking a cheap-store receiver, you can even buy a receiver that covers the frequency in question (and more) for less than the Sports Ears product. Dick Smith for example still carries an TV-audio receiver, catalogue number A-4289 ($29.98 - appears to be a Digitor branded Tecsun). Over the years they have carried a number of similar units, I have an older model which is essentially identical internally, you often see them at garage sales or ham radio trash and treasure days for about $5.
Yet another way to receive the signal is to build a converter that mixes the 70.2 MHz up to something in the FM broadcast band. The region around 100 MHz is empty by design for LIPD devices, like toy radio microphones. It is a simpler matter to build a 30 MHz oscillator and mixer to shift the signal up to 100.2 MHz. In fact I found by experiment that most cheap FM broadcast receivers have such poor front-ends just placing them near a strong 30 MHz signal will cause them to mix up 70.2 MHz to 100.2 MHz internally. A simple overtone (3x 10 MHz xtal) oscillator held near a junk store FM receiver made a test signal at 70.2 MHz easily detectable with the receiver. A packaged 30 MHz TTL oscillator + a battery is an instant converter with a suitably average FM broadcast receiver. (This trick even works on my VR-500 because its LNA is broad as a barn door and fairly weak IP3 wise too.)
Can I Use The "UMPS" Channel Myself?
Well, technically it is very easy to build an FM modulated flea-power transmitter for 70.2 MHz and use your Sports Ears receiver around the house. Just take the typical "FM Microphone", "Wireless Bug" or "Sound Feeder" and retune it to operate at 70.2 MHz. Legally this is probably slightly more illegal than LIPD devices on the FM broadcast band. Obviously taking such a device to the game would be a very dumb idea, but around your home the chances of the signal being seen at any distance are quite small unless you build a good antenna for the transmitter. One wonders why you would bother hacking over a perfectly usable FM broadcast band transmission device, perhaps even capable of high-fidelity stereo transmission in the case of a sound feeder, and locking it down to a fixed-channel, mono uncommon receiver like the Sports Ears. A similar argument might equally apply to Sports Ears in the first place...
Summary
Sports Ears works well. It is fun, pretty much as described in the marketing hype, definitely worth a try if you are attending the games. The implementation method is perhaps a bit morally questionable, especially as the units are arguably overpriced, but the consumer is informed about the proprietary nature of the system and its limitations - the FAQ is quite clear in that respect. The consumer however is not informed that alternatives are available and this was my main motivation for writing this article.
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