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Good Website explaining the slant on SSTV

Using google search is good and we often come across a lot of junk, but here is a good site explaining the SSTV slant and how to resolve some issues.

SSTV Image Slant Adjustment:

and a sample of the WWV time signal

What is slant?

Slant is a distortion of the received image due to differences between sample clock of the PC at the transmitter and frequency of the sample clock of the PC at the receiver. This distortion skews the received image as shown in the figure below.

What causes slant?

Slant is caused because analog SSTV protocols such as Martin or Scottie use an asynchronous transmission protocol. This means that the clock in the soundcard of the receiving PC is not synchronized to the soundcard clock of the transmitting PC. Because of the difference in clock rates, the effective display width of each pixel received will be effectively different from the intended width. The result of this is that with each line the image will be increasingly displaced. If the transmit clock is faster than the receive clock, the image will become slanted towards the right, and if the transmit clock is slower than the receive clock the received image will appear to slant to the left.

How bad can it get?

In MMSTV, the decoder uses the sync pulses at the start of the picture to align the start of the picture to the top left corner of the display. The horizontal sync pulses are then ignored (except if lock is lost and re-lock option has been selected). The time discrepancy between the transmit and receive clocks causes a displacement error that accumulates for the entire picture.
In the previous figure, note that the slant was due to a frequency difference of only 1 Hz (90 ppm) between the TX and RX clocks (nominal clock frequency 11,025 Hz.). Martin and Scottie modes each have 320 active pixels per line x 3 (R,G,B) and after taking into account sync width, this is about 1000 pixels per line. Since there are 256 lines per frame that means there are 256,000 clock cycles per frame.
An error of 90 ppm will then cause an error of 256,000 x 90 x 10-6 = 23 pixels. Over 256 lines, this is a skew of more than 5 degrees which is pretty impressive for a 1Hz error.

How can I correct slant?

You can manually compensate for the error in your PC clock with a clever alignment procedure built in to MMSSTV.

Additionally MMSSTV can be set to automatically correct for small differences in the sample clock of the transmit and receive ends of the communications link. This process is known as Auto slant. The Auto slant process measures the average time between sync pulses and since this should represent a known number or pixels (320 active pixels per line plus the width of the sync pulse) the clock rate of the original signal can be estimated and the difference can be compensated for. For greater accuracy this averaging is performed over a number of lines.

Even if you have calibrated MMSTV to compensate for the offset in your sound card clock, you may find the signal you are receiving has not been compensated so it is generally best to keep this feature turned on.

September 2017
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