Why Do Satellite Receivers Scan as they Do?

[cyberham]

I notice my Ali chip receivers like GT Media, GSP microHD, Openbox S9 always blind scan first horizontal transponders then followed by vertical transponders. Sometimes on a given satellite, they scan from lowest frequency to the highest frequency. But on other satellites, they scan from the highest frequency to the lowest. Why the difference in scan direction?

Whereas my Edision Mio+ receiver always scans vertical transponders first followed by horizontal transponders. As far as I know, it scans from lowest frequency to the highest.

 

[one_db_compression]

Educated, simplified guess? SMOC. Simple Matter of Code. Someone programmed it this way.

More complex guess? It is dependent on the silicon and whether the scan is a big software routine (e.g. - code sets a frequency and iterates through a range) or it is hardware-supported (e.g. - code sets a range in the tuner and the tuner itself iterates through the range).

It is a bit like the turbo code discussion from a few months ago... there, the silicon definitely supports the encoding, but your particular driver doesn't have the logic to twiddle the registers and logic in order to enable the function. Additionally, different silicon will implement hardware-assisted routines with different levels of efficacy.

Long answer... if you know the specific tuner chip model and have the programming guide, you can look at a bunch of open source code to best understand how and why particular developers did what they did. More specifically, the design and programming errata documents for particular silicon and/or code is sometimes more enlightening because you are more apt to have disclosure of limitations and hardware and software bugs.

Source: 30 years ago, ago I was a board-level digital designer and programmer coming out of electrical engineering school.

Everyone else's mileage may vary.

 

[Titanium]

Basically, all is based on the preference of the coder. With that said....

If you are noting the scan direction to be related to the band (I.E. Ku and C), this is because satellite receivers tune an IF (Intermediate Frequency), but what you see displayed is a calculated downlink frequency (IF +/- the LO frequency). The satellite receiver scans in the same direction, but it appears to be swapped when the LO frequency is added or subtracted.

Example: on KU-Band, an IF of 950 + an LO of 10750 = 11700 MHz transponder frequency (low end of the band) The higher the IF, the higher the transponder frequency.

On C-Band, an IF of 950 - an LO of 5150 = 4200 MHz (high end of the band). The higher the IF, the lower the transponder frequency.

 

[cyberham]

...If you are noting the scan direction to be related to the band (I.E. Ku and C)...

Example: on KU-Band, an IF of 950 + an LO of 10750 = 11700 MHz transponder frequency (low end of the band) The higher the IF, the higher the transponder frequency.

On C-Band, an IF of 950 - an LO of 5150 = 4200 MHz (high end of the band). The higher the IF, the lower the transponder frequency.

I think this is what I'm seeing!