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You can't. It's a 'feature'. I have some relatives who preferred their old 4:3 programmes stretched to fill the whole 16:9 screen. Somebody will be along shortly telling you that doing this would be incorrect etc. All I know is that all previous non-YouView Humax boxes could do this but YouView does't support this... 😞
Humax and all the other Youiview box suppliers (eg Huawei) have to conform to the Youview specification - So many of the features on previous Humax PVRs are not available on Youview boxes. Many TVs will allow you to change the picture aspect if you want to. There are many user requests on the Youview community forum - which is probably the best place to lobby for any changes.
I thought Youview boxes passed the signal in the format that the channel transmitted which may be the original format it was filmed in or the format that the channel opted to transmit in.