Zero cross detect - switching audio

[dhouston]

The delay will vary with the frequency of the audio but if you always switch on the falling edge, you will be slightly ahead of ZC and can add a delay if needed. The app note I referenced details how to calculate the delays for 60Hz - you can adapt it to your audio frequencies.

[HankMcSpank]

Aah...tks Dave.

I should prpbably point out that I'm not after a perfect 'zero' switch...just ahem 'close to zero' (I guess what I really want is an 'avoid switching at signal voltage peak' type of switch!)

[dhouston]

I should prpbably point out that I'm not after a perfect 'zero' switch...

Perfect would require calculating the difference between Logic 0 and Logic 1 vs ZC for each frequency and for both rising and falling edges. Close enuff is probably using Logic 0 on a falling edge to trigger the switch.

[HankMcSpank]

Close enuff is probably using Logic 0 on a falling edge to trigger the switch.

:-)

Bert, ok...just sussed, where you came up with the 333us number!

1.5khz maximum audio frequency expected = 1/1500 = 660us period .....therefore half a period would be 333us.

so 'zero cross to signal peak' (90 degrees) at that frequency would be about 150us..... if I can get within 10% of zero @1.5Khz I'd be very happy.

If I run the PIC at 16Mhz, I reckon I ought to be get reasonably close to zero - even at the maximum expected audio frequency of 1.5Khz - in my mind, 15us is a chunky amount of 'window' to get an audio IC switch after the PIC's comparator has toggled?