Re: Morphy Richards DRM27024

DAB sounds worse than FM wrote:



I haven't read all of the posts in this sub-thread, and I've only read Jamie's comments where you've included the quotes in your reply, but here's how I see the noise issue, and it won't cancel:

The L+R signal is at baseband. The L-R signal is upconverted to be centred at 38 kHz, so the noise on the (L+R) and downconverted (L-R) signals is different, so the stereo channels would be reconstructed as follows (N1 is the noise on the L+R signal and N2 is the noise on the L-R signal):

Left = 0.5 [(L+R+N1) + (L-R+N2)] = 0.5 x (2L + N1 + N2) = L + (N1 + N2)/2

Left = 0.5 [(L+R+N1) - (L-R+N2)] = 0.5 x (2R + N1 - N2) = L + (N1 - N2)/2

But (N1 - N2) won't reduce the noise level because noise is a zero mean random process, so there's just as much probability of both N1 and N2 being negative as there is of them being positive.

Hummm. Well in theory I don't quite agree that N2 would not cancel.

I agree in theory with Jamie's point that N2 is derived from exactly the same source, the stereo sub carrier. Therefore the N2 noise component in each channel, coming from exactly the same source, should be exactly equal and opposite. The fact that it is random, should be irrelevant, because it should in theory be exactly the same for each channel. Two things that are the same, are the same, regardless of whether their identical source was a random source. Therefore adding the channels together should cancel out N2.

However I'm trying to point out that what works in theory, doesn't always work in practice. I'm not sure exactly what could happen to stop this theory from becoming reality, but have been speculation on a few possibilities.

Perhaps in a cheaper radio, the version of L-R used for the left channel is not actually identical to the one used for the right. In theory they should be identical, but are they in reality?

An other idea, if the radio doesn't work that way, but uses the multiplexing method, then L-R doesn't actually come into it. In theory the multiplexing method should generate the same results as decoding the stereo sub carrier. In practice, however perhaps this is not true in an imperfect receiver. Perhaps if the timing of the multiplexing is not perfect, then some other effect is preventing N2 from cancelling.

I'm not sure exactly how you got the result you got, where a radio with one speaker produced more noise when in stereo mode. In theory I don't think that should happen, however I assume that you have no reason to lie about it, so there must be some reason why that happened. To me the best explanation seems to be that the theory doesn't necessarily work in an imperfect receiver.

Richard E.
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Relevant Pages

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