Wireless – Will Digital kill the Analogue Star?

A personal view from Pyers Easton

In a word, until the laws of physics are bent, no!

It’s very much a ‘horses for courses’ decision because, despite what some sales people will have you believe, digital is not necessarily better. There are advantages to be gained by undertaking the signal processing in DSP, and there are advantages, further expounded below, to using an analogue wireless transport medium. I believe that a hybrid of DSP for audio processing and analogue for the wireless connection gives the best of both worlds.

The main issue (apart from TX battery life) with digital is that the threshold is higher. Meaning you need more signal in the digital domain than analogue. This is down to pure physics, analogue FM is a single RF carrier that is modulated in the frequency domain, whereas digital modulation is basically a noise signal that takes up the entire channel bandwidth (usually 200kHz, see Fig 1).
Depending on the exact form of the digital modulation, typically the threshold is 10dB or more worse than the analogue equivalent in the same bandwidth. This will mean that a digital transmitter must be 10 times or more the power output of its analogue counterpart for the same operating range.
Whilst this may not be an issue in controlled environments, such as a fixed studio installation, in the case of OBs and any range critical application analogue wins hands down.

Another significant drawback is that digital TX signals develop what are known as ‘shoulders’ due to the RF amplifiers not being perfectly linear. These shoulders extend above and below the desired signal and can sterilise at least one adjacent channel. This will in turn reduce the number of channels of wireless that can fit in a TV channel without significantly reducing the range due to co-channel interference from the shoulders of the adjacent channel.

Analogue FM systems do not suffer from this issue as it is just a single carrier signal. With modern FM systems, such as the new Wisycom ‘LinEar’ TXs and narrow band RX (100kHz instead of 200kHz) you can stripe TXs every 200kHz without worrying about intermods, thereby getting 38 channels in an 8MHz TV channel without any loss of range, in fact the narrower bandwidth of the RX means that the sensitivity is actually enhanced by 3dB, the same as doubling the TX power!

 

Figure 1. shows an analogue and a digital signal overlaid. As you can clearly see the digital signal takes up a significant amount more bandwidth, and also creates noise right out to the edge of the spectrum plot. The shoulders are also clearly visible and illustrate how it would sterilise the channels above and below. The addition of another digital mic on an adjacent channel would see a significantly reduced range.

 

 

 

 

An additional issue that plagues digital receivers is that, in order to maintain and ensure linearity in the RF amplifiers, they mostly use an AGC (automatic gain control). The result of this is that if you are receiving a distant digital TX and someone walks close to the RX antenna with another TX on a different frequency, the RX will wind down the gain in the RF amp and the distant TX may drop out.

So, for the foreseeable future, I think it’s safe to say that analogue is here to stay.

 

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