One of the biggest headaches of NBTV is the requirement of very good low frequency performance, Klaas Robers mentions on the NBTVA website, a figure of around 2Hz being ideal. Few audio devices have a frequency response down to such a low figure, even 20 Hz is often dubious. It's not so much the electronics but more often the record/playback media employed, whether it be tape, PC soundcards, whatever. Surely there's a way to negate this?
There is. While I'm stuck in the Maldives (I cannot get home due to no flights - COVID-19) I've come up with this idea...
USB ports can handle digital I/O at amazing speeds so why not make use of that? Combined with a Terminal Emulator program we have a fast (up to a point) serial digital I/O.
What this requires is a USB to TTL converter, I have access to two types. A USB to RS232 'dongle' is a possibility too...if fast enough.
Let's start with the software required on the PC. Forget Microsoft Hyperterminal, it ignores certain control codes and doesn't record them. I use Realterm, a freebie download with no ads etc. and goes up to 921,600 Baud which we need here. It takes a bit of getting used to, but it's worth it.
You'll need some form of USB-TTL conversion device, FTDI (UK) do 'cables' that convert USB into TTL serial data, just as if it had come via a RS232 to TTL device (a MAX232). Website here...
https://www.ftdichip.com/Products/Cable ... Serial.htm Also distributed by the usual big companies RS/Farnell/Element 14 etc.. You'll be needing either TTL-232RG-VSW3V3-WE or TTL-232RG-VREG3V3-WE. Add your own connector to the bare wire ends, I used a 9D so I can use it on more than one external device. In this case all that is needed is TX, RX and GND. There are other sources too...Google it I guess...they're also available direct from the FTDI website via the previous link, as is the driver, on the same page (VCP drivers).
Add to that a DC-coupled video a-d/d-a conversion and an AC-coupled audio a-d/d-a conversion and you're in business!
So based on the above premise here's my plan...probably better explained in diagrams at this stage.
Click images for higher resolution version.
I'm not so keen on Butterworth filters for NBTV, they tend to introduce over/undershoots on sync pulses for example, but OK for audio, I may change them when I can experiment.