Ask HN: Why is Amazon's NFL “live stream” 30 seconds behind cable?

In the analog days it was hard to delay a television signal, practically you would have to put it onto a film loop, tape loop or possibly some kind of analog delay line. Back then, live TV was really live with almost no delay.

In the digital age, it is easy to delay a television signal. In fact, if you watch the same broadcast through different paths (OTA TV, cable, satellite, FM Radio, ...) you will see that the broadcasts are perceptibly out of sync with each other.

In the case of internet streaming, there is often a large delay introduced by the encoding process. Remember AMZN has to produce streams at several different bit rates, possibly supporting different compression methods for different devices. They encode it in one place and send it out over a content distribution network (at least to other AWS nodes) Each step in the journey adds some latency, particularly due to the use of buffers.

Football is particularly challenging because of the high-complexity and high-action and rapid cuts. Variable Bit Rate helps a lot because you can take advance of times when they show graphics and talking heads to send data for the complex scene that is next to come. That adds latency (the coder has to be 10+ seconds ahead of now) but really helps with image quality, reliable streaming, low bandwidth requirements, etc.

Most people won't notice the out-of-sync unless they watch two streams at once or if they do real-time sports betting. Thus they prioritize the other characteristics, which customers will notice.

For that matter, even cable/satellite feeds run 8-15 seconds behind real time (easily noticed if you listen to a non-delayed radio broadcast at the same time).

Radio broadcasts may themselves be delayed for ratings tracking.

Money, of course. Why do you think such large sums are bid for screening rights? Sports fans want to know what's going on as quickly as possible, both to feel like they're part of the action and because they may be gambling on the outcome.

Architecture. They're probably writing every X seconds to an S3 bucket, updating that bucket content to a cloudfront URL which then servers requests from end users. You can see how this pipeline could incur tens of seconds of latency over actual real time. As long as the latency is constant and in the tens of seconds, it qualifies as a live stream in my book

A very simple technical reason. https://en.wikipedia.org/wiki/HTTP_Live_Streaming