since most broadcast HDTV signals are around 19Mbps
Most Canadian services backhaul their signals to the BDUs over fibre - the 19Mbps number is specific to OTA broadcasts. So in reality, the HD signal Shaw, Bell or Rogers gets from the broadcaster could be anything from uncompressed baseband (1.5Gb/s) to something between 30 and 50Mb/s MPEG-2.
You need to review the whole acquisition->broadcast chain to appreciate how much compression is in play throughout.
HDCAM (144 Mb/s) and HDCAM-SR (440 Mb/s) are by far the most popular HD tape delivery formats for television programming in the broadcast world. However, with the recent Japanese Earthquake/Tsunami crisis, a number of key factories were affected causing huge tape shortages. This has made a massive push on the industry towards file-based delivery methods.
But even further up the chain, XDCAM-HD (50Mb/s) is emerging as one of the most popular broadcast production camera formats. This is disc-based recording media but there is now a memory-card based version (XDCAM-EX) at 50Mb/s also.
Uncompressed HD (1.5Gb/s) production exists but is not too common and obviously expensive given all the resources required to manage it.
And depending on the storage servers (Evertz, Omneon, Nexio) used by the broadcaster for ingesting of media (tape or file), an additional level of compression is applied. XDCAM-HD @ 50Mb/s is again emerging as a new common standard for storage of media for broadcast. It also happens to be edit-friendy (Final Cut Pro, Avid).