CPU cycles for video encoding

548 Views 2 Replies 3 Participants Last post by TomCat, Oct 22, 2012

tigerwillow1

Discussion Starter · Oct 22, 2012

I'm wondering, when using HDMI, does the DVR have to decode the compressed video then re-encode it for the HDMI output, or does it just pass the original encoded video through? Then as a follow-on, I'm wondering if some outputs and/or video formats take more CPU cycles and if this could possibly explain why some users report their DVRs as "dog slow", while others are "snappy"?

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P Smith

· #2 · Oct 22, 2012

Yes and no.

No re-encoding in term of picture parameters (resolution, color, fps, etc) but transforming to different type of electrical signals.
Process of the transforming to HDMI (TDMA signaling) or component or composite executing by specialized chips or parts of that "CPU" (what is not a CPU but SoC with convenient CPU [MIPS] and specialized subsystems for decoding, decompressing, etc).

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TomCat

· #3 · Oct 22, 2012

There is indeed a decoding step in the DVR, but the output of that decoding (for HD) is full-blown 1.5 gbps uncompressed digital video, which is precisely what HDMI was designed for. So, no, no reencoding is necessary nor performed. A hardware DAC extracts the analog outputs from that same video.

Decoding MPEG-4 video is done in hardware, so there are few if any CPU cycles involved in that task beyond auxilary support. Rescaling and interlacing/deinterlacing is also hardware-based. So, no, none of that contributes significantly to CPU overhead causing DVRs to be sluggish. There are plenty of reasons why; this is not one of them.

It's a terrific question, but I'm afraid there is no connection.

And few, if any, are snappy. The general consensus is that all DTV DVRs are either sluggish on occasion, or often. I have yet to hear the term "snappy" used for a DTV DVR. If you want snappy, you pretty much have to go to DISH, who has long-ago proven it is possible (implying that DTV engineers either are not trying hard enough or are just not smart enough).

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