CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Beginning balance, shares at Apr. 30, 2018	1,648	55,689
Beginning balance, value at Apr. 30, 2018	$ 2	$ 55	$ 614,810	$ (559,129)	$ 55,738
Series E preferred stock dividends			(2,162)		(2,162)
Cumulative-effect adjustment to accumulated deficit pursuant to adoption of ASU				2,739	2,739
Common stock issued under Employee Stock Purchase Plan, shares		40
Common stock issued under Employee Stock Purchase Plan, value			114		114
Exercise of stock options, shares		334
Exercise of stock options, value		$ 1	1,157		1,158
Stock-based compensation expense			622		622
Net loss				(3,412)	(3,412)
Ending balance, shares at Oct. 31, 2018	1,648	56,063
Ending balance, value at Oct. 31, 2018	$ 2	$ 56	614,541	(559,802)	54,797
Beginning balance, shares at Jul. 31, 2018	1,648	55,990
Beginning balance, value at Jul. 31, 2018	$ 2	$ 55	615,040	(558,351)	56,746
Series E preferred stock dividends			(1,081)		(1,081)
Common stock issued under Employee Stock Purchase Plan, shares		40
Common stock issued under Employee Stock Purchase Plan, value			114		114
Exercise of stock options, shares		33
Exercise of stock options, value		$ 1	143		144
Stock-based compensation expense			325		325
Net loss				(1,451)	(1,451)
Ending balance, shares at Oct. 31, 2018	1,648	56,063
Ending balance, value at Oct. 31, 2018	$ 2	$ 56	614,541	(559,802)	54,797
Beginning balance, shares at Apr. 30, 2019	1,648	56,136
Beginning balance, value at Apr. 30, 2019	$ 2	$ 56	613,615	(560,605)	53,068
Series E preferred stock dividends			(2,162)		(2,162)
Common stock issued under Employee Stock Purchase Plan, shares		47
Common stock issued under Employee Stock Purchase Plan, value			187		187
Exercise of stock options, shares		127
Exercise of stock options, value			431		431
Vesting of restricted stock units		28
Stock-based compensation expense			1,254		1,254
Net loss				(3,594)	(3,594)
Ending balance, shares at Oct. 31, 2019	1,648	56,338
Ending balance, value at Oct. 31, 2019	$ 2	$ 56	613,325	(564,199)	49,184
Beginning balance, shares at Jul. 31, 2019	1,648	56,238
Beginning balance, value at Jul. 31, 2019	$ 2	$ 56	613,395	(563,769)	49,684
Series E preferred stock dividends			(1,081)		(1,081)
Common stock issued under Employee Stock Purchase Plan, shares		47
Common stock issued under Employee Stock Purchase Plan, value			187		187
Exercise of stock options, shares		53
Exercise of stock options, value			173		173
Stock-based compensation expense			651		651
Net loss				(430)	(430)
Ending balance, shares at Oct. 31, 2019	1,648	56,338
Ending balance, value at Oct. 31, 2019	$ 2	$ 56	$ 613,325	$ (564,199)	$ 49,184