I don't know what flatline is, but some possib;e clues until we get more help here...

https://books.google.com/books?id=hDgEAAAAMBAJ&pg=PA128&lpg=PA128&dq=Which+Power +Macs+have+an+AV+in+Port&source=bl&ots=SzplYi7XkK&sig=RnWh1ctJAfgeonO4EuYRmZMoFT Q&hl=en&sa=X&ved=0ahUKEwj40Ijgq4XOAhUDzGMKHSsmCGYQ6AEIbDAN#v=onepage&q=Which%20P ower%20Macs%20have%20an%20AV%20in%20Port&f=false

One of the better old PowerMacs with analog to digital AV was

the PowerMac 8500 series. One I had showed promise; then I

donated it along with huge pile of working stuff to a charity thrift.

• Power Macintosh 8500 - wiki

The Power Macintosh 8500 is a model of Macintosh personal computer designed, manufactured and sold by Apple Computer from 1995 until 1997. Billed as a high-end graphics computer, the Power Macintosh 8500 was the first Macintosh to ship with a replaceable daughtercard. Though slower than the 132 MHz Power Macintosh 9500, the first-generation 8500 featured several audio and video in/out ports not found in the 9500.

In fact, the 8500 incorporated near-broadcast quality A/V input and output and was the first personal computer to do so, but no hard drive manufactured in 1997 could sustain the 18 MB/s data rate required to capture video at that resolution. Later, special "AV" hard drives were made available that could delay thermal recalibration until after a write operation had completed. With special care to minimize fragmentation, these drives were able to keep up with the 8500's video circuitry.

• PowerMac 8500 (aka workgroup server 8550)

http://lowendmac.com/1995/power-mac-8500/

However the 'flat line' reference escapes me - in this perspective.