Through friends in the Russian science community, I was able to get the raw telemetry from Venera-13 and 14. I funded its recovery from old tapes and conversion to TIF format. It consisted of multiple transmissions with considerable noise. 
For redundancy, the radio system used two completely different schemes: pulse code modulation at decimeter wavelength, and an old system of pulse-position modulation at centimeter wavelength (with orthogonal coding).
Comparing the two versions, one can reconstruct a remapping that repairs many of the bad pixels in the PPM version. This is especially valuable near the end of the transmission, where the PPM signal was much better than the PCM signal, which became very noisy.
The pixels were 10 bits: 9-bit video and a parity bit. I realized that some noisy sections were just shifted by a bit or two, which can be fixed. This was good on Venera-14, where the widest image was late in the transmission and never-before-seen lines could be recovered.
Once all these repairs and some others were done, the versions of the transmissions were aligned and a "best-of" master signal was created.
The next problem is to linearize the raw camera signal. There are algorithms for extracting camera response from images, and this was made easier by the presence of a grey-level ramp that the camera scanned during the retrace interval.
Finally, the linearized image was converted to sRGB. We can compare the original Soviet processing from the 1980s (left) with the new processing (right).
