ATSC update

Hi again. While making ATSC recordings, I ran into a problem that I think you should be aware of. Even with 100% perfect reception, your final decodes may be corrupt.

The moral of this post: Use RAM drives to record your IQ samples! SSD's might also be a good idea.

How I found out:
As I described in the first post, it takes a lot of bandwidth to record the IQ samples necessary to capture a TV station. Surprisingly, it takes 32 MiB/sec for 8 MHz of bandwidth, even though you're doing 16-bit samples. It seems that it should be 8 Megasamples/sec * 2 bytes/sample = 16 Megabytes/second.

If you use a magnetic hard drive to record IQ samples and some of your TV recordings are corrupt and won't play, chances are good it's the hard drive not keeping up.

On a side note: you may have also noticed that 8 MHz looks cleaner in the waterfall than 7 MHz, although 7 MHz can work. 6 MHz is just too narrow, because of the IF filters that round off the edges.

Using RAM drives, I had been limited to ~1500-1800 MB available RAM on one computer, which was only enough for about a minute. The only way to get more space was to use a hard drive. But hard drive recordings never played very well. I began to wonder if the recordings somehow got bad after a certain amount of time.

I tested this by taking several very large IQ recordings (of different stations.) Some were recorded to an 8 TB external drive I purchased recently, others to a very fast internal magnetic SATA drive, and one more on a 5 or 6 GB RAM drive (on different computer.)

Of the three, the only one I could actually watch was the one recorded to the RAM drive.

This is proof enough that filesize won't mess up the decoder; apparently the hard drives were causing samples to be dropped.