IC-R75 RTTY Anomaly

From The RadioReference Wiki

Last year, in response to some correspondence on the ICOM R-75, Klaus Betke sent the following:

1. If two options what is the BFO/CIO offset options for CW.  Can it be varied and if so how?
In 10 Hz steps from 600 to 900 Hz. I have set it to 700.

2. RTTY. Is it fixed at the usual (hi-tones) 2210Hz?
I admit that I don't understand the settings. "RTTY mark frequency" can be selected
from 1275, 1615 and 2125 Hz.
Apparently this is also the center frequency. In a further setup entry, you can
select RTTY shift between 170, 200 and 425 Hz. I have not yet found where this
has an influence on.

Recent emails and some experiments with/by Patrice Privat have clarified and revealed what is considered a design problem with this receiver in respect of the RTTY mode.

Amateur Radio RTTY

To recap, the digi-monitor can employ three different modes in a receiver to produce audio tones to drive the decoder. The decoder may have a fixed or variable centre frequency (CF). The CF is that frequency, in simple terms, below which incoming tones are considered as being MARK, and above which are considered SPACE (as per ITU Normal).

  • USB-Tune the receiver to the wanted frequency LESS the decoder CF.
  • CW-Tune the receiver to the wanted frequency. The BFO/CIO must equal the decoder CF. The setting in the R75 is called CW Pitch in SET mode.
  • RTTY-Tune the receiver to the wanted frequency. The receiver will produce tones based on recognised "standards". The decoder CF must equal the standard selected or provided.

In the world of amateur radio digimodes use is made of the terms high tones and low tones. "High tone" standard are used almost exclusively in the American continents (and many other areas). There is also a "low tone" standard more popular in Europe - that is from an American book [RTTY Today by Dave Ingram K4TWJ]. Published 1984 things may have changed in that everyone may now be using high tones.

In some receivers (eg my NRD545) there is no option and the value is fixed at the more marketable (in USA) high tones. In the R75 both high and low tones are offered, plus a third which will be discussed later.

Radio amateurs tend to have a fixed value for the MARK and a variable value for the SPACE being

  • MARK+170Hz for NARROW shift, and
  • MARK+850Hz for WIDE shift.

Let's look at the "standards" table for NARROW shift

                Mark    Center  Space
High tones      2125    2210    2295    R/Amateur standard
Low tones       1275    1360    1445    R/Amateur standard
**** tones      1615    1700    1785    "Maritime" ?

In the case of the R75 ICOM differentiates between the three by giving the RTTY MARK value in user SET mode (RTTY MARK= 2125,1275,or 1615). In my case, for both the NRD545 and the NRD535, JRC offer the center frequency (there is only one setting) of 2210Hz.

A Third Option?

But what is third option **** tones? For want of a definative name one might call it maritime tones. 1700Hz, as a centre frequency, has been a popular value on maritime modems. Also if one uses the RTTY/NAVTEX text modes of JVcomm32 and MScan Meteo Pro the center frequency is fixed at 1700Hz.

So far so good but now we meet the anomaly in the R75 design. Here the selected RTTY MARK= selection does NOT result in a MARK+85Hz centre frequency offset of the Mark/Space tones but the selected MARK value becomes the centre of the received pair ie the output produced in fact is

                Mark    Center  Space
High tones      2040    2125    2210        R/Amateur standard
Low tones       1190    1275    1360        R/Amateur standard
"Maritime"tones 1530    1615    1700

To compensate for this the R75 must be detuned by -85Hz, or the decoder CF readjusted to equal the appropriate centre from the R75 "actual" table.

Finally, and further to Klaus's "In a further setup entry, you can select RTTY shift between 170, 200 and 425 Hz. I have not yet found where this has an influence on." comment, this seems to be confirmed in our recent observations. Also interesting to note it does not offer 850Hz.

Observations for the above were achieved using Spectrogram V5.

Thanks to Klaus and Pat for their respective inputs.

From WUN0905 'Digital Review' column May 2003 by Day Watson (c) Worldwide Utility News