New API 1.95 improves zero IF image rejection

[DaveB]

Having got API 1.95 working by deleting the registry entry as suggested I was then able to check whether there was any improvement in the zero IF image rejection.

I'm very pleased to find that there is a significant improvement, so that a signal 60 dB above the noise floor produces only a weak image putside the FM broadcast band - which if it falls in band is masked by the in-band noise.

My reference signal is Heart 102.8. With a centre frequency of 105.8 an image will appear on 108.8. With this latest API it is only 5dB above the noise floor on 108.8, but the noise floor within the FM band is somewhat higher and moving the centre frequency so that the image falls on 107.4 - which is a clear channel - it is for all intents and purposes absent.

This is a very significant improvement for HDSDR in particular as HDSDR has a very good recording scheduler - but if you use the manual I/Q adjustment it only works on the I/Q signal after the point at which the signal is recorded and so playing a file back in another program you find a strong IF image only 45 dB down.

I dare to suggest that the SDRPlay correction is slightly better than the one in SDR# build 1361 - the last build that can be fully utilised by SDRPlay.