Both analogue and digital TV equipment are designed using dedicated video chips designed compliant to the international professional analogue and digital TV standards. The system can operate from the recoding stage to the viewing stage without any computer processing.
General purpose computers do not contain hardware video chips. They use software to program the CPU and GPU to mimic the various digital TV standards. If a person looks carefully at the screen they can observe the programming artefacts.
According to the publicly available from Sigma, the Sigma chip contains hardware video chips designed compliant to the standards; there is no software or hardware post processing involved.
You are dealing with technologies designed 30 years apart. You don’t state whether you are using NTSC or PAL. NTSC has inherent design issues that PAL resolved.
All TV technologies are manufactured with component tolerances usually 10%. The Sony Trinitron picture tube is the best of all the analogue displays. If there is an issue with the Sony TV, the chances are it is a calibration issue.
The only way you can confirm the issue is using a virgin rip DVD and viewing the VOB file for artefacts. If you are still experiencing artefact issues there is nothing in the Sony TV that is user serviceable. I own two, have the service manuals for both and I am proficient in electronic servicing. I would say based on my experience that the Sony TV components have drifted past the 10% component tolerances.
If your DVD player has the same 10% component tolerance drift it is possible that the two are compatible with each other.