Remember Vinyl and tapes? Then along came CDs – and the majority agree that was a big leap forward in audio quality. Then came MP3s and we were sold on how digital progression could give us less for more. Same great sound but in less bits, so we could get more on our computer or download faster. It wasn’t long before people realised that although MP3 offered data reduction improvements with its obvious benefits like music streaming, there was a penalty in audio quality.
In the broadcast industry, an advance to digital technology has opened up many a passionate debate. DAB was promoted as the future 20 years ago, but it’s poor coding technology and the desire to squeeze more stations in the same amount of digital bandwidth has seen a reduction in quality for the listener compared to analog FM. The same debate rages for IBOC/HD and other new broadcast systems.
The perception that digital is better has recently crossed into the analog FM transmitter industry. Although the authors of this article and the author of the referenced white paper acknowledge that a correctly designed direct to channel digital synthesis of the FM carrier dramatically improves the audio and RF performance of the FM signal, we look to highlight, like MP3, that just because a transmitter generates it’s signal digitally (DDS) it doesn’t always make it perform better.
The picture above illustrates a clean FM transmitter as shown in the report below.
The attached white paper was written as a study of the noise performance of Direct Digital Synthesis (DDS) and analog FM broadcast transmitters, comparing performance when un-modulated, modulated with single frequencies and modulated with audio. The tests are carried out on three transmitters, two implementing DDS: a PTEK FM150ES and a Zhongchuan Digita ZHC618F-100W, and one analog: a BW Broadcast TX150. Although the commonly held belief is that DDS provides considerably greater performance compared to analog transmitters, the white paper demonstrates that this is not the case for the devices tested.
The tests show that for the two DDS devices chosen, the RF spectral output of the transmitters does not conform to the ETSI EN 302 018-2 european harmonised standard for FM broadcast transmitters, making them unsuitable for use in many countries. In some cases they may also become dangerous to use as they cause interference in bands reserved for aircraft automatic landing and communications systems. Throughout all of the tests, the analog BW Broadcast TX150 conforms fully to the standard. The results are plotted as frequency spectra given with 1MHz bandwidth to see spurious signals close to the carrier, and at 50MHz to see signals further from the carrier.
The picture above illustrates two different transmitters with the same modulation.
One is digital and marketed as state of the art, while one is analog and may be considered older technology. Which is which? Click the report link below to see if you guessed right.