Electronic – How to introduce delay to a signal

If the delay down the copper wire is less than the encoding delay though, surely bits will be being received too quickly at recipient end, faster than they can be decoded into a digital stream which could be buffered?

I think the key point you're missing is that it's entirely possible for more than one bit to be "in flight" on the wire at any given time.

For example, if the wire is 100 m long, and the velocity is 192 x 10⁶ m/s, and the bit rate is 100 Mb/s, then 52 bits of data will actually be "on the wire" at any given time. The receiver, however will only be aware of the 1 bit that is actually arriving at the receiver at that instant.

If the transmitter is sending bits at 100 Mb/s, then the receiver must receive and decode these bits at 100 Mb/s. The length of the wire changes the latency time between these two events, but it has nothing to do with the rate at which the receiver must deal with the incoming data.

Usually the receiver doesn't deal with the incoming bits one at a time, doing calculations at 100,000,000 operations per second. Instead it simply queues the bits up into something like a shift register, and then operates on them at a much lower rate, maybe 12.5 million operations per second, but operating on full bytes with each operation (or even at slower rates, but operating on larger data words).

OK, let's assume you're talking about an analog signal that has a bandwidth of about 5 MHz, and you need to maintain a signal-to-noise ratio of about 72 dB, or about 12 bits of resolution.

One direct way to delay the signal is to digitize it, write it to a memory and then read it out again at a later time.

To capture a baseband signal with a bandwidth of 5 MHz, you'll need to sample at a minimum of 10 M samples/s. To delay this by up to 10 seconds, you'll need 100 M samples of storage. Call it 200 MB for convenience. SDRAM would be an excellent fit.

You need an ADC, the SDRAM, and a DAC, plus some sort of control interface. You can get fast MCUs, DSPs and FPGAs that all have the necessary SDRAM and I/O interfaces. The choice of what kind of chip works best for you depends on whatever else (if anything) you might want it to be doing.