Try changing the quality setting on your slide to High Quality. Does that help?
Is this a standard Captivate caption or one you have created?
Like Rod, I'm not sure to understand your question. You are talking about which Text Captions? And what do you mean by the 'Alpha Channel'? Did you apply some transparency anywhere yourself? Another tip: please let us know which version of Captivate you are using. There are quite a lot of versions used all over, and UI and possibilities are quite different.
@ Rod many thanks setting quality did the trick. Setting to high killed the alpha on the halo captions.
May I just thank you Lilybiri and you Rod for being so helpful with this crash course of mine - after monday I shall be able to learn at a more leisurely pace. The IT issue, I have still to receive a report which I shall post but in the meantime it seems they will accomodate a training area.
First prize would have been a case of file sent to learners' desktop.
Many thanks once again.