GNU Radio does have blocks that can subtract streamed data in real time

GNU Radio does have blocks that can subtract streamed data in real time, though there isn’t a single block literally called “Subtraction”.

You can achieve subtraction cleanly in a few ways, depending on your situation:

---

🧮 1. Add block with negative gain (most common method)

• Block: Add (or Add Const if subtracting a constant)
• If you want:
  [
  \text{Output} = A – B
  ]
  Connect stream A to input 0, and stream B to input 1, then set the gain of input 1 to –1.
• Or, more simply: feed B through a Multiply Const block with value –1, then into the Add block.

🧭 Typical use: removing a reference signal or background level from a measurement stream.

---

🧮 2. Sub block in the “Math Operators” category (if installed)

• Some GNU Radio distributions (especially 3.10+ with standard blocks) include:
  • Sub (Subtraction)
  • Multiply, Divide, etc.
• This block directly outputs:
  [
  \text{Output}[n] = \text{Input0}[n] – \text{Input1}[n]
  ]
• Great for sample-by-sample subtraction of two streams.

📍 If you don’t see this in your installation:

• It may be under “Operators” or “Math”.
• Or you can add the equivalent with Method 1.

---

🧮 3. Custom subtraction with “Python Block” or “Expression Filter”

• Use the Single Input/Multiple Input Expression block or embedded Python:

·         out[0] = in0[0] - in1[0]

• Useful for more complex operations (e.g., subtracting a running average or calibration reference).

---

⚡ Important Notes

• Both input streams must have identical sample rates and be synchronized (aligned in time).
• If one stream lags behind, you may need a Delay block to line them up.
• Subtraction happens sample-by-sample, not block-wise (unless you use a Vector-based block).

---

✅ Example in radio astronomy:
If you’re using two antennas with SDRplay RSPduo and want to remove common-mode RFI, you can subtract the reference channel from the signal channel using:

[SDR 0] --->(A)---+

                  |---> Add block ---> Output = A - B

[SDR 1] --->(B)-MulConst(-1)-+

---