630x348 is nearly poststamp size. So dont expect high quality especially if played full screen on a hd monitor.
As Ann mentioned here, your resolution is tiny compared to what we use nowadays. For example, FullHD is 1920 x 1080px.
With your resolution, as you watch the video playing in your video player at full screen it's going to look very bad.
There's no way around it unless you use higher resolution, FullHD being pretty much a minimum nowadays.
Aside from the small desired resolution (640x348 pixels), can you tell us more about the bitrate you have chosen to encode the file with?
Please include a screenshot of your sequence and export settings.
What device are you viewing it in?
It is a low resolution, my colleagues ACP already told you and it is. Try to change the size or resolution so that you can improve the quality of your video because you have high resolution photos and videos.
- Select the Sequence in the Project panel.
- Go to the menu Sequence / Sequence Settings...
- Change to Custom in Editing Mode.
- Change the Full HD 1920 x 1080 in Frame Size.
- Press OK
- Readjust the footage from the Effect Controls panel / Motion tab (position, scale, etc.)
Upscaling SD in Premier to HD and then exporting to SD again. Dont think that will do much good.