Recent technology has enabled truly high-quality HD On 6 Mbps connections. I'm thinking with overhead and the obvious inability to reach peak bandwidth, you'd want a minimum connection of 8-10 Mbps.
I reach peak bandwidth on my 8 Mbps connection all the time. You only don't reach peak bandwidth if you're on a provider who doesn't give you what they advertise (for example provisioning DSL at rated speed but using ATM).
Anyhow, depending on how much quality you want, you can get HD streams pretty compactly.
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