M2TECH Joplin - Manual (Page 37)

JOPLIN
384
k H z /32bit ANALOG-TO-DIGITAL CONVERTER
REVISION P R B – SEPTEMBER 2012
Copyright © 2012, M2Tech Srl
37
M2Tech
www.m2tech.biz
The Joplin is provided with 4 curves for tapes, 2 for NAB and 2 for CCIR/IEC.
9.3.1. CCIR/IEC for 9.5cm/s and 19cm/s (3¾ ips and 7½ ips) and CCIR/IEC for
38cm/s (15 ips) (IEC1 and IEC3)
CCIR (Comité Consultatif International pour la Radio) was an European Committee similar
to NAB. They proposed an equalization for reel-to-reel tape recorders which was adopted
by most European manufacturers. It has been said by many that the CCIR equalization is
better than the NAB one. Anyway, the choice of one or the other standard is only an option
when recording, as some tape recorders have both equalizers. For playback, the choice
depends on the standard used to record the tape. For the record, all commercial pre-
recorded tapes available on eBay or other auction sites are made using NAB.
9.3.2. NAB for 9.5cm/s (3¾ ips) and NAB for 19cm/s (7½ ips)
NAB proposed two equalization curves for tapes, one for 3¾ ips (curve B) and one for 7½
ips (curve A), plus an amendment for 15 ips which, anyway, was suggested to avoid (7½
ips was the preferred speed for tape recorders used in broadcasts). The Joplin offers both
curve A and curve B.
9.4. Acquiring with equalization
When no equalization curve is used, then the rightmost led of each VU-meter bar turn on
right before clipping (that is, when the input signal is -1dBFS). Should the input signal
further increase its amplitude, the ADC saturates and the “CLIP” warning is displayed on
the display. User should avoid getting the “CLIP” warning appear at any time during
acquisition.
When an equalization curve is used, then the VU meter is not consistent with the effective
risk of saturation, as it indicates the signal downstream the equalization. User should then
pay attention to “CLIP” warning appearing even when the bar is not completely lit. As a
general rule, ADC clipping may occur when using equalization at a VU meter level such as
the 4-5 rightmost leds are still off (this indicates a post-equalization level of -5/-8dB).
9.5. Interfacing a cartridge or a tape head
The Joplin has an input impedance of 47 kOhms, that is a standard value that fits all line-
level sources and also MM cartridges. On the other hand, MC cartridges and also tape
heads may need a different load impedance.
Due to the small size of the Joplin back panel, no room was available to add some
impedance matching switches. Should an impedance matching be necessary, we suggest
users to use a “T” adaptor and RCA plugs in which tip and ring are connected with a
resistor or a capacitor of the required impedance value.