SIR

SIR ("Signal-to-Interference Ratio") is a new feature developed to help automatic offset hunting in VOYAGER (see details in the VOYAGER help).

Most DXers are familiar with SNR ("Signal-to-Noise Ratio") but SIR is something new. When trying to find potential hits (i.e. audible signals) based on any given offset, we usually have to deal with many interfering stations on the same frequency. This is why SNR is not useful for this task.

We can also study the plain signal power on the desired offset but this is not optimal, either. Such an algorithm would not pick the time spots we are especially interested in but most likely the times when there is generally lots of signal power on that frequency (especially at the nighttime and/or when the antenna preamp is enabled). Instead, we are interested in the times when the relative signal power on that desired offset is the highest, compared to the signal power of the highest offset peak(s), So, when defining SIR in this way, it will also cover the cases where the desired offset itself is the highest.

The SIR value for a given offset is

• the signal power of that offset divided by the strongest offset peak on that frequency,

unless the studied offset itself is the highest; in that case, the SIR value for this offset is the signal power divided by the 2nd strongest offset peak. This is probably easiest to understand with examples. The SIR value can change rapidly over time; therefore, the better resolution you use in SCANS, the better results you will get in SIR-based offset hunting:

• SIR 0.2: The offset of interest is 5 times weaker than the currently strongest offset on the frequency.
• SIR 1.0: There is another offset peak on the frequency which is exactly as strong as the one we are interested in.
• SIR 3.5: The offset of interest is 3.5 times stronger than the next strongest offset on the frequency.

In most cases, the most powerful signal peak is also the one which is audible (if any). However, this may not always be true, but normally it can be kept as a general guideline. There is not much information available under what circumstances a "lower peak" can dominate in terms of audio. However, when searching for "potentially legible" time spots for an offset, you can just scan the recordings and keep a list of the highest SIR values which can then further studied with traditional listening.

It will be interesting to see what the lowest useful SIR values are for picking up an ID. Quite likely it can be very low on regional channels if there is only one additional dominant station present (and it happens to have a short break e.g. at TOH). On the crowded graveyard channels it is quite likely that the lowest SIR with audio will be much higher (probably SIR > 1.0).

The problem with classic spectrum displays is that it is often impossible to see the difference of the peak powers just from the spectrum colors along the timeline. In the future, SIR values can be used for evaluating the "relative offset power".

HOW TO

SIR is controlled and handled automatically by JAGUAR; you just have to be aware of this feature.