diff --git a/Documentation/DocBook/media/v4l/vidioc-g-modulator.xml b/Documentation/DocBook/media/v4l/vidioc-g-modulator.xml
index 80167fc4483d..96e17b344c5d 100644
--- a/Documentation/DocBook/media/v4l/vidioc-g-modulator.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-g-modulator.xml
@@ -78,6 +78,12 @@ different audio modulation if the request cannot be satisfied. However
 this is a write-only ioctl, it does not return the actual audio
 modulation selected.</para>
 
+    <para><link linkend="sdr">SDR</link> specific modulator types are
+<constant>V4L2_TUNER_SDR</constant> and <constant>V4L2_TUNER_RF</constant>.
+For SDR devices <structfield>txsubchans</structfield> field must be
+initialized to zero.
+The term 'modulator' means SDR transmitter in this context.</para>
+
     <para>To change the radio frequency the &VIDIOC-S-FREQUENCY; ioctl
 is available.</para>