Experiences with CH38

In the UK this year, we've been in the process of switching over our radio frequencies used for PMSE (Programme Making and Special Events) from channel 61 - 69 (790 - 862 MHz) to channel 38-40 (606.500 - 613.500 MHz). With access to channels 61-69 completely stopping in the end of 2012.

There's been plenty of posts and discussions about the politics of this switch over, but I've not read very much about the real world examples of switching over, and the experiences. So I thought I'd summarise what I've been through in the past year, whilst working on two big television dramas (Accused and Mr Selfridge) that required multiple radio microphones sets ups.

Allocated Frequencies

In the UK the radio frequencies for radio microphones are well co-ordinated by the JFMG. For Channel 69 (the old channel that was regularly used for location recordists) there was a set of 14 frequencies that were available to use and share within the channel, and various sub-sets of which were intermodulation free. This meant that it was fairly straight forward to get 7 or 8 radio channels up and running together, where ever you were and whatever brand of radio mics you owned; as each manufacturer made sure that at least some (if not all) of these listed frequencies were available.

In the migration to Ch38 (the new channel for the shared frequencies) the decision has been made by the JFMG to not specify the frequencies available within this channel - the reasoning being that its now possible for different manufactures to design their products to fit in more intermodulation free frequencies. Even though the JFMG has a list of suggested frequencies for Ch38, this means that each  manufacturer has a different frequency list...and suddenly it has become a lot harder to work with different equipment.

Complete Flexibility

Realising this, it made sense to purchase a radio mic system that was completely tuneable to any frequency, such that it would be possible to adapt to any combination of radio microphones and to set up the radios so that they worked in whatever situation. Most manufacturers now offer some radios that are completely tune-able, but each still have their own list of recommended intermodulation free frequencies. I'd been a user of micron radio microphones for a number of years, I decided to carry on using them, especially as they were now offering a tuneable LCD controlled set. I've a set of four of these now, that are set up in a pair of DDH3 racks with a pair of dipole aerials.  (I'm not going to review / comment on the radios themselves here...but may do later as I've found pro's and cons to them).

On the drama Mr Selfridge this year, it's been necessary to have a large number of radio microphones - up to eight on certain scenes, so this meant hiring in additional microphones to my complement of 4 in Ch38 and 2 in Ch69. So I naturally hired in microns...only to find that the hire company had changed the set of intermodulation free frequencies from the standard micron set. If I had not opted for a flexible system, this would have been a huge stumbling block as the two sets of frequency tables did not have any common frequencies (see the table below - micron specific is the standard set, micron modified is the set that came with the hired microphones). For info, the numbers in italics are the memory locations that I've set up within the micron LCD Rx and Tx, so that I can now swap between the standard frequency set, the 'modified set' from the hire company and the JFMG shared frequencies.

Ch 68: Licensed		Ch 38: Licensed			Ch 38: Licensed		Ch 38: Licensed
		Micron Modified			Micron Specific		JFMG Shared
Ch 69: Licensed		606.125	10	Micron A	606.125 A		606.600	0
854.900	Senn Tx 2	606.500	11		606.675 N		607.500	1
855.275		607.250	12	Micron B	607.375 Y		608.150	2
855.900		608.750	13	Micron C	607.925 B		609.150	3
858.650	Micron 2	609.625	14	Micron D	608.300 C		609.950	4
860.400		610.550	15		608.900 D		610.550	5
860.900	Micron 1	611.250	16		609.325 P		611.250	6
861.750		612.300	17		611.825 E		612.300	7
Ch 70: Un licensed		613.150	18		612.300 F		613.150	8
863.725	Private Comms	613.500	19		613.000 H		613.500	9
					613.525 T
					613.850 M

Ch 38 Itself

I've been using channel 38 since November 2011, and I've had mostly successful experiences but there does feel to be a definite difference to channel 69 - although I can't objectively quantify these feelings! My gut feeling is that the range is not quite as good, but this may be because of the ever increasing RF signals that as flying round set, from the wevi, the walkies, mobile phones, remote focus controllers and even the remote control for the smoke machine.

I'm also finding Ch38 to be more unpredictable that ch69 was. Previously, once a set of frequencies were up and working with a set of 4 or more radio microphones, this would remain to be the case - not withstanding location anomalies. But with Ch38, I can frequently find that one of my regular 4 radio channels will start under performing, with reduced range, increased background noise / compansion artefacts - a change of frequency usually sorts this out - but often this can be as quick as from one shot to another. It can be quite frustrating.

4 October 2012