- 50Hz hum
- RF interference from various sources
- Bad ground/ ground loops
- gain/impedance mismatching
Mind you, the audio input on your sound card is usually already a disaster having bad or failing ground contacts. Next the pc has analogue and digital circuitry, there's a cable to your TRX and there it is susceptible to RF. Not to mention the many ground loops etc..
I have been successfully using Spectran, a freeware audio analyser programm by I2PHD and IK2CZL.
When the audio is directly linked to my sound card input you get the below result (volume settings at zero):
My sound card is an old 16bit Soundblaster compatible type. Any current 192KHz/24bit High-End sound card will lower the noise floor by some additional 10 -15db. For the required measurements this old 16bit card has proven to bemore than adequate.
How to get a 'clean picture'
- Use properly shielded cable
- Install an audio isolation transformer
With the RX Audio Interface inserted (as near at the sound card input as possible), you get the following result: