No external video with Quicktime

I tried to capture a VHS cassette using either a MovieBox DV - Pinnacle Systems or a Canopus - TwinPact 100 through firewire connection.

No driver is needed but two adaptators Firewire 400->800, Firewire 800->lightning.

While iMovie successfully gets both the audio and video signals, QuickTime says "No signal".

Both QuickTime 7 pro or QuickTime Player display the name of the connected hardware (Twinpact 100 or Pinnacle DV).

Both they automatically resize the window to the appropriate dimensions (QT 7 requires a relaunch though).

Both reluctantly display a black screen and I wonder why.

I though that most of the video import/export options of iMovie were performed by QuickTime or directly inherited from it.

Therefore I am puzzled that, unlike iMovie, QuickTime 7 and X are unable to display the image while they obviously get the signal (automatic window resizing).

Is there an explanation for that ? Is this a bug ?

This was checked on two Macs, one with Mavericks and the other with El Capitan.