10 Mbps is more than enough for video. Xfinity tv is built on a technology called HLS. Apple, Google, and Netflix also all use this. The top bitrate offered by xfinity.tv is exposed in the HLS manifest. Take an example HLS manifest for mr robot. Here we see:
This indicates the top bandwidth is 5899312 bits per seconds (or ~6Mbps). That's a pretty standard 1080p streaming bitrate, and well within a 10 (or in your case 50) Mbps bandwidth including someone else browsing or gaming.
HLS is delivered over TCP, not UDP. If you are seeing "pixelated/blocks" showing up (called macroblocking) its because your playback device has selected a lower quality stream.* Now this could be for a huge variety of reasons:
- 1. You are not actually getting 50Mbps from the modem. Certainly possible, and this could be caused by a whole host of issues like: bad signal to the CMTS, an overloaded network segment, a misconfiguration on the modem etc. A quick speedtest on a wired (not wifi!) device would show this. Comcast is actually pretty good about hitting advertised speeds. If your not getting them advertised speed contact them (there is an internet chat option which is pretty good).
- 2. You are having issues with your wireless router speed. This is extremely common with older third party routers, they just can not handle the packets per second the bandwidth requires and die.
- 3. Your PC can't keep up. This is also common. If the decoder drops frames because the CPU is to busy you will drop bitrates
- 4. The video itself has transcoding or source errors. This is possible, but much less likely. If before "packaging" for HLS delivery the video has errors this can occur. If you are adventurous you can look in the network diag console to see what bitrate you are pulling (the information is exposed in the url).
- 5. Their CDN can't keep up. Probably the least likely of all, but still possible
None of these have to do with needing 50Mbps. If your not getting somewhat close to that 50Mbps (within about 10%) you should be on Comcast's customer support like white on rice. In any case really you should be, but din't just assumed incompetency.