[RTTY] Noisy computer line in

[Kok Chen]

> On Jan 15, 2017, at 9:56 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> Also... this may sound stupid... but make sure you are actually feeding the
> decoder off of "line in" and not off of a microphone that may be plugged
> into the PC. 

This probably happens more often than you think :-).

I have many times received email that cocoaModem was not decoding well even with clean loud signals.  I tell them to make sure the sound interface is selected for line-input rather than computer microphone (many Macs come with built in microphones), and usually get an "oops" reply.

When sound card levels are set up properly, a computer microphone that is place at the receiver's speakers can work surprisingly well.  However, if the microphone is more than a foot from the speaker, the room noise (fans, keyboard, dogs and children, etc) starts to become a factor (think S/N ratio).  

A microphone that is further away from the speakers can also generate multipath from sound that reflects off other objects and the walls of the room.  An RTTY symbol of 22ms is equivalent to the time it takes sound to travel about 25 feet.  If the distance from the speaker the wall and then back to the microphone is more than 10 feet, the modem receives the equivalent of moderately severe multipath :-).

Just turn off the volume from the speakers, and if the RTTY decoder also stops decoding, it is likely that you are receiving through a microphone instead of the line-input.

73
Chen, W7AY