[OZAPRS] FT-290R + Signalink USB

[Ray Wells]

Nick/All,

Let's not forget that the frequencies are 1000Hz apart for 1k2 packet 
(1200/2200Hz) and frequency response at those two frequencies may be all 
important for successful decoding of packets.

Assuming the correct (6dB/octave) pre-emphasis at the tx, the rx 
discriminator output will show high tone close to twice the level of the 
low tone and, for post de-emphasis, the two tone levels will be close to 
the same. Those situations are normal. Things can get a bit off the 
rails, however, if speaker audio is taken for the TNC/modem.

By way of an example, the very first radio I used for packet, about 
1985, was a Multi-750 (FM/SSB/CW). The "speaker" audio had been tailored 
so severely (presumably to make the audio sound half decent on SSB) that 
the level at 2200Hz was way short of what was required for the TNC. 
Enough that I could decode about one packet in one hundred. As a 
stop-gap measure I made an external RC filter to provide locally 
pre-emphasised audio for the TNC (TNC220). Later, I took post 
de-emphasis audio from inside the radio, via an emitter follower.

Modern modems and perhaps software modems are probably more tolerant of 
incorrect audio levels than TNC's based on the Exar 2206/2211 pair where 
the decoder chip would fail to decode if the high tone was even 1-2dB 
lower in level than the low tone. The TNC2 and variants (MFJ1270, e.g.) 
all used the Exar pair.

Whenever I have decode issues, the first thing I check is the relative 
tone levels with a CRO. There are plenty of software oscilloscopes 
available these days so almost everyone can have access to a CRO. I 
recommend you look at this piece of Freeware - Visual Analyser 2011. To 
quote WA8LMF, "This *FREEWARE*  program, written by an Italian 
electrical engineering PhD candidate at the University of Rome, emulates 
a whole stack of HP audio test instruments in a single 5 MB exe file."

It is an impressive array of audio test equipment.

Ray, vk2tv

On 10/02/12 08:14, Nick (VK3TY) wrote:
> Hi there,
>
> I used to used my FT290R Mk I on Packet back in the early days.  The
> interface cable went directly to the multi-pin plug without having to insert
> or bypass anything. I'll be back on air with AX.25 signals shortly - using
> my Yaesu FT-290R and original interface a cable.
>
> The issue of frequency response is somewhat a mute point when only two
> (mark/space) tones are used.
>
> The IRLP link I provide does have an issue using a direct link to the
> microphone and audio output.  The radio circuits have de-emphasis and
> pre-emphasis can have a significant effect on the full voice frequency
> range.  That is why with IRLP, audio is take off post the discriminator and
> mic. audio is fed in at another point leaving the normal microphone and
> speaker points alone.
>
> Nick - VK3TY
>
> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of Peter Dodd
> Sent: Thursday, 9 February 2012 10:37 PM
> To: Australian APRS Users
> Subject: Re: [OZAPRS] FT-290R + Signalink USB
>
> Hi,
> I have a FT-290R but have not used it with Signal link (because I don't have
>
> one).
>
> I recall seeing that a capacitor possibly need removing from the mic cct. to
>
> use the radio for packet (something about frequency response).
>
> Not sure if I can find article.
>
> Regards
>
> Peter
> VK7KPC
> ----- Original Message -----
> From: "Liz"<edodd at billiau.net>
> To:<OZAPRS at aprs.net.au>
> Sent: Thursday, February 09, 2012 10:07 PM
> Subject: [OZAPRS] FT-290R + Signalink USB
>
>
>> Has anyone got this combination of an FT-290R and a Signalink USB
>> operational?
>> I don't think reinventing the wheel is worthwhile, so I thought I'd ask.
>>
>> I also realise that the radio is old, and no one may have one in use.
>>
>> Liz
>> _______________________________________________
>> OZAPRS mailing list
>> OZAPRS at aprs.net.au
>> http://lists.aprs.net.au/mailman/listinfo/ozaprs
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