Initialism of pulse code modulation. (the system of encoding sound on a telephone for use in the PSTN, using a compander)
"The engineer explained that converting analog audio to PCM was necessary before transmitting the voice call over the traditional phone network."
per calendar month
"The landlord agreed to charge a fixed rent of $1,200 pcm regardless of how many days are in each month."
Initialism of per calendar month: used in lease arrangements, stating that costs are to be paid on a fixed date every month.
"The landlord informed me that my rent is charged pcm, so I need to transfer the full amount by the fifth day of each month without fail."