Recording the past. Capturing raw analog signals

Here's a hypothetical. If you were to travel back in time, what would be the best way to record analog broadcast from radio and television without conversion?

You could convert the signals into another format —such as TV into digital video in a codec like ProRes or compressed h.264. But that's converting it from analog to a digital format. Is there a way to digitally capture an analog signal? Similar to Laserdisc technology?